like Inge mentioned, the document no has been used. you need to check a few spots
1 check FA ledger entries to find if the no. has been posted, if the posting is correct
2 check the no. series related to FA posting for the last used no.
the thing to look out for is other user might manually changed document no. and messed it up. e.g.
no. from 1 to 10
1, and 2 used and someone picked 5 to post, so it goes on leaving a gap somewhere. another user hit the issue and pick other random no.
you could export the fa ledger entry to excel, sorting the no and figure out where the gaps are
