If you can enter values directly into the table you should be able to enter
them via the form. I'm starting to understand that the main record is a
bill of lading, and that the invoices are created separately from that.
That is, the bill of lading is created from existing invoice records. If I
do not understand correctly (or even if I do), some detail about the
database structure would help. For instance, you say you added a new field
to the form. Number field? Indexed? Duplicates allowed? Does the error
message arise when you are attempting to add values to this field?
You say the invoice number is part of a relationship to another table? What
other table? How is it that the two tables are connected by invoice number?
That is, what data are stored in the two tables that the thing they have in
common is invoice number?
The question is not what you should be looking for in the form, but rather
what is needed in the way of tables and relationships.
I agree that if there is more than one invoice per bill of lading you should
use a related table, not a second invoice number field. There is no
guarantee you won't have a third invoice number some day. Sometimes it is
OK, IMHO, to risk having a structure that does not perfectly allow for extra
data. For instance, in most cases one middle initial or middle name field
is enough. For the occasional person with two or more, add both to the same
field. This is not one of those situations. You really should consider
seriously a related table.