Outside of syntax errors, the most common cause of things "not appearing" is a lack of anchors and/or dimensions. A UI object must have at least one anchor (using SetPoint) and either explicitly set dimensions (using SetSize/SetWidth/SetHeight) or dimensions implied by using multiple anchors (eg. TOPLEFT and BOTTOMRIGHT).
And speaking of syntax errors, make sure you have some kind of
error display at all times while writing addons. The default UI hides them by default, and even if you turn on the "Show Lua Errors" option it still has some deficiencies -- a big one being the fact that it can't show errors that happen too early in the loading process, which is exactly where syntax errors happen.
Or if you have Lua installed on your computer, you can run "luac -p your_file_here.lua" from the command line to check for syntax errors. If you're using an IDE with support for Lua, it may have this feature built in.