Use a dynpro number less than 9000

What causes this issue?

You attempted to assign an invalid screen number.
Screens 0001 - 8499 are for SAP applications. Those from 8500 to 8999
are for SAP Partner developments, whilst screens 9000-9999 are for
customers' own developments.

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?

Please support customers and SAP Partners in their development by
observing this convention.

Error message extract from SAP system. Copyright SAP SE.