Why can't I change my name and I waited for 90 days and when I try to change it said that {"message":"Form is not valid.","errors":{"change_username":"This form should not contain extra fields."}} please help me.
I believe that only happens if you have an account created with a linked sign in (Google, Facebook). Those don't have a password, which is expected during a name change, so won't work.
You'll need to get a password assigned to the account. I think that might require contacting Support directly using the option in the menu.
Why can't I change my name and I waited for 90 days and when I try to change it said that {"message":"Form is not valid.","errors":{"change_username":"This form should not contain extra fields."}} please help me.