Help me fix what did I do wrong ?

-- grid-demo
-- Grid testing.

term.clear

-- Variables
button_lc = 1
button_rc = 2
exit_  = "q"

w,h = term.getSize()

-- 3x3 grid
grid = {
    0,0,0,0,0,0;
    0,0,0,0,0,0;
    0,0,0,0,0,0;
}
gridColumnWidth = 6

-- Draw the grid
function drawGrid()
    for i=1, #grid do
        term.setCursorPos(1,i)
        term.write( string.rep("*",gridColumnWidth) )
    end
end
drawGrid()
function SetTile(n) grid[ n+#grid ] = 1 end
function DetTile(n) grid[ n+#grid ] = 0

term.setCursorPos( (w-string.len("Hold CTRL+T to exit.")) + 1, h )
term.write("Hold CTRL+T to exit.")

-- Main program
while true do
    local event, button, x, y = os.pullEvent()
    if(event == "mouse_click") then
        if(button == button_lc) then
             if(x >= 1 and x <= gridColumnWidth and y >= 1 and y <= #grid) then
                SetTile(x)

             --drawGrid()
             term.setCursorPos(x,y)
             term.setTextColour(colours.white)
             term.setBackgroundColour(colours.white)
             term.write("D")
    end
        elseif(button == button_rc) then
            if(x >= 1 and x <= gridColumnWidth and y >= 1 and y <= #grid) then
                DetTile(x)

            --drawGrid()
            term.setCursorPos(x,y)
            term.setBackgroundColour(colours.black)
            term.setTextColour(colours.white)
            term.write("*")
   end
        end
    end
end