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You had your code in a function called
myCallback which was in the HeadHTML. However, in the Actions inspector, you had set a click to run
untitledFunction() which is empty.
You need to move only the code inside
myCallback (not the whole
myCallback function) into
untitledFunction(). Once you do that, it will work provided you add quotes around your ID like this:
("Contador"). If you don’t add quotes, the code will look for a variable named
Contador which, of course, does not exist. IDs are strings, not variables.
If I were you, I would look at the documentation that the others have linked to because you have to learn a little bit more about the basics to understand what you’re trying to do here.