Cardinality &2 of Association &1 does not match to the ON-condition

What causes this issue?

The cardinality &V2& of the association &V1& does not match to the given
ON-Condition:
If the key-elements of the targeted data source are all compared with an
EQUAL Operator with a corresponding element in the current data source,
a cardinality of TO-1 is expected.

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?

This warning can be suppressed by putting the pseudo comment
/*+[hideWarning] { "IDS" : [ "CARDINALITY_CHECK" ] }*/
in front of the view definition or the association definition.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.