Entry of an external document number not allowed

What causes this issue?

Your system is configured in such a way that document numbers are issued
by the system. The field <LS>Document number</> must be left empty.

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?

Delete the number in the field <LS>Document number</>.

Error message extract from SAP system. Copyright SAP SE.