Hello Amit,
This is purely a number range issue. And, One of the document which is gong to be created automatically maintained as External Number Range. We just need to find out which document might be that.. Here are some suggestions to figure out the issue from where it is hitting.
I suggest you to look at Number Ranges that are maintained in EWM. and check, which one's are maintained external Number range. And, Cross verify is that document getting generated here during our process. If yes, then, Just try changing it as internal number range and they try again. Till we come to conclusion.
Another way is, Analyzing the created documents. that means, Do we have Plain inbound deliveries created, If yes, then, no issue with Inbound deliveries. Then check, HUs are getting created. If yes, Then, no issue with HUs. So, by adding which element we will come to know the issue is getting. Then, we need to rectify that elements number range issue.
It's only help on how to rectify.
Regards,
Sathish