my subform accurately pulls in value when I select from a combo box on the main form, but when I click ANYWHERE on the form after that (I’m trying to add a record to the subform) I get the error message that the “index / primary key can’t contain null value” Could any possibly offer a suggestion where my index might be null/blank as I have gone into the main forms record source & made sure there aren’t any null / empty fields.

The problem with the null key may be on the subform rather than the form. Check to see if the subform's key is getting properly populated for all new subform records.