[Fizzlemizz]

Given you haven 't added your startup code, I may be misunderstanding the question but I think this may be closer to what you're trying to do,

local name, addon = ...
 
function addon:CreateKeyboard()
    local Keyboard = CreateFrame("Frame", 'KeyUIMainFrame', UIParent, "TooltipBorderedFrameTemplate") -- the frame holding the keys
--    _G["Keyboard"] = Keyboard
    tinsert(UISpecialFrames, "KeyUIMainFrame")
 
    Keyboard:SetWidth(1099)
    Keyboard:SetHeight(448)
    Keyboard:SetBackdropColor(0, 0, 0, 0.9)
    Keyboard:SetPoint("CENTER", UIParent, "CENTER", -300, 50)
 
    Keyboard:SetScript("OnMouseDown", function(self) self:StartMoving() end)
    Keyboard:SetScript("OnMouseUp", function(self) self:StopMovingOrSizing() end)
    Keyboard:SetMovable(true) -- make the keyboard moveable
    Keyboard:SetScale(1)
--    Keyboard:Hide()
--    Keyboard.Close = CreateFrame("Button", "$parentClose", Keyboard, "UIPanelCloseButton") -- Close button is on the Controls frame
--    Keyboard.Close:SetSize(25, 25)
--    Keyboard.Close:SetPoint("TOPRIGHT", Keyboard, "TOPRIGHT", 0, 0)
--    Keyboard.Close:SetScript("OnClick", function(self) print(self:GetParent():GetName(), KBControlsFrame:GetParent():GetName()) end)
 
    addon.keyboardFrame = Keyboard
 
    return Keyboard
end
 
function addon:CreateControls()
    local Controls = CreateFrame("Frame", 'KBControlsFrame', UIParent, "TooltipBorderedFrameTemplate")
--    _G["Controls"] = Controls
--    tinsert(UISpecialFrames, "KBControlsFrame")
    local Keyboard = addon.keyboardFrame
    local Mouse = addon.MouseFrame
    local modif = self.modif
    Controls:SetBackdropColor(0, 0, 0, 1)
    Controls:SetPoint("BOTTOM", Keyboard, "TOP", 0, -2)
--    Controls:Hide()
 
    local function OnMaximize()
        Controls:SetHeight(100)
        Controls:SetWidth(Keyboard:GetWidth()) 
        maximizeFlag = true
    end
     
    local function OnMinimize()
        Controls:SetHeight(26)
        Controls:SetWidth(Keyboard:GetWidth()) 
        maximizeFlag = false
    end
 
    Controls.Close = CreateFrame("Button", "$parentClose", Controls, "UIPanelCloseButton")
    Controls.Close:SetSize(20, 20)
    Controls.Close:SetPoint("TOPRIGHT", -2, -2)
    Controls.Close:SetScript("OnClick", function(s) KeyUIMainFrame:SetShown(not KeyUIMainFrame:IsShown())  end) -- Toggle the Keyboard frame show/hide
 
    Controls.MinMax = CreateFrame("Frame", "#parentMinMax", Controls, "MaximizeMinimizeButtonFrameTemplate")
    Controls.MinMax:SetPoint("RIGHT", Controls.Close, "LEFT", 0, 0)
    Controls.MinMax:SetOnMaximizedCallback(OnMaximize)
    Controls.MinMax:SetOnMinimizedCallback(OnMinimize)
    
    if maximizeFlag then
 
        Controls.Refresh = CreateFrame("Button", nil, Controls, "RefreshButtonTemplate")
        Controls.Refresh:SetSize(24, 24)
        Controls.Refresh:SetPoint("TOP", Controls.Close, "BOTTOM", 0, 4)
        Controls.Refresh:SetScript("OnClick", function(s) addon:RefreshKeys() end)
 
        Controls.Slider = CreateFrame("Slider", "KUI_Slider1", Controls, "OptionsSliderTemplate")
        Controls.Slider:SetMinMaxValues(0.5, 1)
        Controls.Slider:SetValueStep(0.05)
        Controls.Slider:SetValue(1)
        _G[Controls.Slider:GetName().."Low"]:SetText("0.5")
        _G[Controls.Slider:GetName().."High"]:SetText("1")
        Controls.Slider:SetScript("OnValueChanged", function(self) Keyboard:SetScale(self:GetValue()) Controls:SetScale(self:GetValue()) end)
        Controls.Slider:SetWidth(224)
        Controls.Slider:SetHeight(20)
        Controls.Slider:SetPoint("BOTTOMLEFT", Controls, "BOTTOMLEFT", 15, 55)
 
        Controls.ShiftCB = CreateFrame("CheckButton", "KeyBindShiftCB", Controls, "ChatConfigCheckButtonTemplate", BackdropTemplateMixin and "BackdropTemplate")
        _G[Controls.ShiftCB:GetName().."Text"]:SetText("Shift")
        Controls.ShiftCB:SetHitRectInsets(0, -40, 0, 0)
        Controls.ShiftCB:SetPoint("TOP", Controls, "TOPLEFT", 26, -84)
        Controls.ShiftCB:SetScript("OnClick", function(s)
            if s:GetChecked() then
                modif.SHIFT = "SHIFT-"
            else
                modif.SHIFT = ""
            end
            addon:RefreshKeys()
        end)
        Controls.ShiftCB:SetSize(30, 30)
 
        Controls.CtrlCB = CreateFrame("CheckButton", "KeyBindCtrlCB", Controls, "ChatConfigCheckButtonTemplate", BackdropTemplateMixin and "BackdropTemplate")
        _G[Controls.CtrlCB:GetName().."Text"]:SetText("Ctrl")
        Controls.CtrlCB:SetHitRectInsets(0, -40, 0, 0)
        Controls.CtrlCB:SetPoint("TOP", Controls, "TOP", 0, -84)
        Controls.CtrlCB:SetScript("OnClick", function(s)
            if s:GetChecked() then
                modif.CTRL = "CTRL-"
            else
                modif.CTRL = ""
            end
            addon:RefreshKeys()
        end)
        Controls.CtrlCB:SetSize(30, 30)
 
        Controls.AltCB = CreateFrame("CheckButton", "KeyBindAltCB", Controls, "ChatConfigCheckButtonTemplate", BackdropTemplateMixin and "BackdropTemplate")
        _G[Controls.AltCB:GetName().."Text"]:SetText("Alt")
        Controls.AltCB:SetHitRectInsets(0, -40, 0, 0)
        Controls.AltCB:SetPoint("TOP", Controls, "TOPRIGHT", -46, -84)
        Controls.AltCB:SetScript("OnClick", function(s)
            if s:GetChecked() then
                modif.ALT = "ALT-"
            else
                modif.ALT = ""
            end
            addon:RefreshKeys()
        end)
        Controls.AltCB:SetSize(30, 30)
 
    else
 
--        if Controls.Close then
--            Controls.Close:Hide()
--        end
    
        if Controls.Refresh then
            Controls.Refresh:Hide()
        end
 
        if Controls.Slider then
            Controls.Slider:Hide()
        end
 
        if Controls.ShiftCB then
            Controls.ShiftCB:Hide()
        end
 
        if Controls.CtrlCB then
            Controls.CtrlCB:Hide()
        end
        
        if Controls.AltCB then
            Controls.AltCB:Hide()
        end
        
    end
    Controls.MinMax.isMinimized = false -- Set the MinMax button & control frame size to Minimize
    Controls.MinMax:Minimize() -- Set the MinMax button & control frame size to Minimize
    Controls.MinMax:SetMaximizedLook() -- Set the MinMax button & control frame size to Minimize
 
    return Controls
end
addon:CreateKeyboard()
local ctl = addon:CreateControls()
 
ctl:SetScript("OnEvent", function(self, isInitialLogin, isReloadingUi)
    if isInitialLogin or isReloadingUi then
        self:Show()
        addon.keyboardFrame:Show()
    end
end)
ctl:RegisterEvent("PLAYER_ENTERING_WORLD")

As an aside, a frame with a name automatically has that name added to the global table as a reference to the frame so:

_G["Keyboard"] = Keyboard

is not required as you can refer to the frame via KeyUIMainFrame

KeyUIMainFrame:Show()

which is why it is best to make sure your frame names are unique by doing something like prefixing them with your addon name.

Blizzard will use the simplest/common names for things (including frames and globals) so words like Keyboard or Controls are not recommended on their own.