What is wrong with MouseButton1Click?

MouseButton1Click is a function. Therefore, you need to integrate it with the correct syntax. You should use it like this:

script.Parent.TextButton.MouseButton1Click:connect(function()
    --Code
end)

or

function Click()
    --Code
end

script.Parent.TextButton.MouseButton1Click:connect(Click)

So, in your case you're attempting to change the text of the TextButton. To do that, we'll use the function, and have it set up like this:

script.Parent.TextButton.MouseButton1Click:connect(function()
    script.Parent.TextButton.Text = "121"
end)

That's all there is to it! So now, this will fire the function, and then change the text to "121".

Anyways, if you have any further problems/questions, please leave a comment below, and I'll see what I can do. Hope I helped :P

-Dyler3

it has to be

script.Parent.TextButton.MouseButton1Click:connect(function()
script.Parent.TextButton.Text  = "121"
end)