No valid waybill stock found, or number &1 is already in use

What causes this issue?

You want to enter a particular waybill number. However, this is not
possible for one or more of the following reasons:
The check digit is incorrect or missing.
The prefix is incorrect or missing.
The waybill number does not exist in any valid number range.
The waybill number has already been used in another document.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.