Cannot change a rule while the rules for this table are in use. (Error 3302)

A record in this table has been changed by another user, or has been changed by you in a separate session of your application, and is currently locked. You cannot modify the rules in a table while a record in that table is locked.

If you have locked the record in a separate session, save the record, and then make your changes. If another user has locked the record, wait a little while, and then save your rule modifications.