The result type of expression &1 must be Boolean

What causes this issue?

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

System Response

How to fix this error?

If the rule line contains an action, the result type of the expression
must be Boolean.
The result of the expression decides whether the action is executed or
not.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.