What does this mean?- in English

[Database Dummy]

The current field must match the join key '?' in the table that serves as the
'one' side of one-to-many relationship....

 

[Allen Browne]

In what context did you receive this message?

Here's a wild guess:
- This error occurs in a subform.
- The subform is bound to a query.
- The query contains 2 (or more) tables.
- You are trying to add a record to the related table.
- There are fields in the lookup table that have a Default Value.

If that is accurate, the problem is triggered when Access uses the Default
Value in the table on the one-side of the relationship and tries to add a
record there. Removing the Default Value from the fields in that table
avoids the problem.

Let us know if that is the context of your error.

 

[Database Dummy]

That's amazing. That's exactly the context - but I hadn't thought about the
default value.