Hi,
1. Please check document type when you posting document no on F-30 (example I assume doc type that used 'DA').
2. Check number ranges that have been used for this document type (T-Code : OBA7) (ex : for doc type DA using number range 04).
3. Check on table BKPF for last document no. that used for document type DA.
4. Change current number on document no. ranges 04 for DA based on last doc no (T-code : FBN1).
5. Try to posting document no using F-30.
Hope this steps can help you.
Regards,
Harris