[Clamsoda]

local function IAmStill()
    local currentSpeed = GetUnitSpeed("player")
 
    if (currentSpeed == 0) then
        return true
    else
        return false
    end
end
 
IAm = {
    still = IAmStill
}

1) "Player" isn't a valid UnitID, "player" is.
2) Make IAmStill local.
3) IAm.still needs to be a function reference, not the return of a function (denoted by ()).
4) I don't condone making IAm global, nor do I condone the way you are calling the function, but I am hoping that it is a crude test and that you intend to localize the table and call it in a better way.

/run print(IAm.still()) should work.