Cannot add acceptance criteria

[Phyllis Hauner Morris]

Why cannot add acceptance criteria for a new feature ticket that is assigned to me?

I get an error message stating that the "Resolution" field must be completed. 

[Nic Brough -Adaptavist-]

That suggests that one of your admins has made the resolution field mandatory in the field configuration scheme.

This is something they should never do - you'll need to find them and get them to fix their mistake.  (Well, ok, there are some edge cases in making project behave very strangely, but this is not the place for them)

There might also be a validator if it was a transition you were trying to use, but it sounds like you were editing, rather than progressing.  If that's the case, the admin needs to remove the validator or put the resolution field on the screen.