Problem with a calculated script for a text box

So i want to put this "event.target.required = (event.value==event.target.defaultValue);", which in another document it stopped puting as required a field once it has been filled. There it had no problem.

But in a recently exported to PDF (so blank slate) document, whenever I try to add this script in the calculated section, it just goes blank.
This error started spreading to my old document and now there I can't add any calculated script as well.... any suggestion on whats going on?