The message means you are trying to create a child record when there is no parent for it and is likely a result of your relationships. Assuming the relationships are correct, then it's likely just a matter of procedure. In other words, don't try to create child records when there is no parent, or design so that it can't happen.
A combo list does not mean there is a record with a value in a field that is bound to the control. Combos used for searches are typically left unbound because it's easy to alter values in the records that you've bound the control to. So the list values are one thing (they typically come from a table/query or value list) and the records where you're saving a selection is something else. The property for the former is known as the rowsource property; the latter is the controlsource property (field that the combo is bound to).
HTH - if not, maybe post the db or refer to a prior post where it can be found.
Last edited by Micron; 09-01-2021 at 10:14 AM.
Reason: clarification
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.