Diagnosis
The business transaction you are performing is &V1&. No external numberrange has been assigned to this transaction in controlling area &V2&,which means that the SAP System cannot assign an external documentnumber.
You can assign a number range in customizing with Environment -> COdocument No. Ranges.

Procedure
Maintain an external number range for this business transaction.
Do not assign external numbers.
Correct any errors in the controlling area entry.