Internal number assignment not defined (please enter number)

What causes this issue?

The system cannot assign a number because internal
<GL:number_assignment>number assignment</> has not been defined in this
case.

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?

Enter a number that lies within the valid <GL:number_range>number
range</> for the document.

Error message extract from SAP system. Copyright SAP SE.