Screen areas '090'/'100' cannot be used together with '091'/'101'

What causes this issue?

You used the screen areas '090' and '091' together. This is not
possible as screen area '090' (customer subscreen - a
screen/notification type) only allows one customer-specific screen area
for each notification type.

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?

If you want to use several customer-specific subscreens in your
notification type, only use screen area '091' in Customizing for this
notification type. The respective screen to be used can be assigned
directly to the screen area '091'.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.