You are saying the form is based on 1 table where all the fields are required and immediately after selecting a combo list item the message pops up? Then you must have code on a combo event that is causing the record to be saved. If I don't have the details right, you need to clarify.
If you are attempting to add a name when this happens, the method usually requires the value to be added and the record updated, which of course you can't because of the constraints you've applied. To use NotInList event, you'll have to move validation off of the table and on to the form.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.