&-& (foreign key definition cannot be specified in full)

What causes this issue?

It is not possible to create the foreign key definition, since all key
fields in the value table (primary key fields) would have to be
assigned to foreign key fields (fields to be checked from the table
itself or possibly from another table). In this case, the assignment
would be via the common domain of the field pairs.

System Response

The system attempted to create a default value for this assignment, but
the corresponding fields from the table to be checked could not be
assigned to all the primary key fields.

How to fix this error?

To be able to activate a relationship, the assignment of primary and
foreign key fields must be complete. Any missing fields must be dealt
with manually afterwards in such a way that the fields to be checked
can come not only from the table itself but also from another table. If
an assignment is not possible, the data design of this table must be
re-examined.

Error message extract from SAP system. Copyright SAP SE.