Table & does not have a foreign key for the primary table

What causes this issue?

No foreign key relationship was maintained between the primary table
and one of the secondary tables of the aggregate.
The secondary tables of an aggregate must be connected with the primary
table of this aggregate by foreign key relationships. This connection
can also be transitive.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Go to the single field maintenance screen of the corresponding
secondary table. Position the cursor to the field for which the foreign
key is to be maintained and go to the foreign key maintenance screen by
selecting the function <LS>Foreign key</>.

Error message extract from SAP system. Copyright SAP SE.