Foreign key &-& (enter valid cardinality)

What causes this issue?

You have entered a semantic foreign key without specifying the
cardinality.

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?

Enter a cardinality. The following cardinalities are possible:
C There may not be more than one dependent record for each record in
the check table.
1 Exactly one dependent record exists for each record in the check
table.
CN Any number of dependent records may exist for each record in
the check table.
N At least one dependent record must exist for each record in the
check table.

Error message extract from SAP system. Copyright SAP SE.