To many associations (cardinality [n..*]) are not allowed here

What causes this issue?

The use of ?to many? associations (cardinality [n..*]) in path
expressions within CDS views is forbidden at the following positions:
WHERE condition: If path expressions with a results set greater than 1
are used within simple comparison operations, the required behavior is
undefined: Does the comparison need to be met for all values or only for
at least one value?
In this case, path expressions with a results set greater than 1 are
forbidden within WHERE conditions.
View extends: When a path expression with a results set greater than 1
is used, the cardinality of the result is modified and hence the use of
theses expressions is forbidden with view extends.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.