Leading doc. type &1 and series &2 already used in digital signatures

What causes this issue?

You have changed the combination of leading document type and series
specified for a number range number. However, this combination has
already been used in digital signatures created for this number range
number and must not be changed.

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?

Reverse the change.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.