I'm sure it's simple too, but I cannot really suggest how to solve it,
since I cannot see your database and have no idea whatsoever how the
appointment and payment tables are set up and how they are related,
nor even what real-life entities these tables represent (other than
their names).

The error means that you have a Relationship defined between the two
tables, and that you must have a valid Payment ID in order to create
an appointment.

Try opening the Relationships window (use the toolbar icon that looks
like three tables joined by diagonal lines); click the View All icon;
and make sure that your relationships make sense. Unless you give
different appointments to different payments, I'm guessing that you
may have a relationship that you don't want.

                 John W. Vinson[MVP]