[Help] Can't add new field to an existing Form

Hi community :)

I need help. I can't add a filed to a window type form on CRM.

In this case, im trying to close a proposal, i click on close and it opens mine windows form to insert the data from the closed proposal (print1), i need to add a new field to that form, but on the form editing menu i can't add a new one (print2) i even can't select any option on the top...

I have full premissions on CRM, i can add fields anywhere, i just can't on that kind of window forms.

Print1

Print2 (new field button its unabled)

Thanks