Hi
Check the below path:
IMG>Financial Accounting (New)>Accounts Payable and recievable>Vendor Accounts>Master data>Preparations for creating Vendor Master data>Assign No ranges to Vendor Account Groups
Assign a no range for your vendor account groups and ensure the number range is also maintained for the no range in t code XKN1
Regards
Sanil Bhandari
PS - Please close your thread, if query is resolved.