The related record(s) in the subform cannot exist before or at the time the
record in the main form is saved.

Open the Relationshps window (Tools menu.) If you have set this up
correctly, you will have created a one-to-many relationship between
tblHeader and tblMain. tblHeader (the main form's table) will be the primary
table in the relationship. tblMain (the subform's table) will be the related
table, so it will have a field that relates to the primary key of
tblHeeader. This relationship will have the Referential Integrity box
checked.

If that is all set up correctly, then you must enter the main form record
*before* you can create the subform records. There can be no related records
in the subform until there is a record in the main form to relate them to.
To prevent accidental records in the related table, I suggest you open
tblMain in design view, select the foreign key field, and set its Required
property (lower pane in table design) to Yes.

I'm concerned about the combo box on the main form. If you are tying the
related record to the main form record on a combo, then the primary field is
probably not unique, which sounds like part of the problem.