Notification &1 not sent to SAP Cloud Notification Service

What causes this issue?

Notification ID <(>&<)>V1<(>&<)> had no recipients with E-Mail Address
configured

System Response

Notifications with recipients who have no E-Mail Address configured will
not be delivered to the SAP Cloud Notification Service

How to fix this error?

If the notifications were intended to be delivered to SAP Cloud
Notification Service, please maintain E-Mail Address for the intended
recipients

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.