Hello Sumantra,
Did you maintained the number ranges for current fiscal year?
If no - go to FBN1 and maintain respective number ranges. If you're not sure which number range to update - check in OBA7 which number range is used for the document type you are trying to post. There also can be errors with document number ranges in non-leading ledgers - you will have to check in transaction FAGL_DOCNR.
Please check the above steps, hope this will help. If it doesn't work, please share screenshots with more details on the issue in particular the number of the error message.
Best regards,
The Wirtschaftsmann