There are a few places that need to be checked to see if any bills already exist with this WIP on:
Check to make sure there isn't any draft batches sat in General Postings.
Check there are no Draft/Unauthorised/Rejected slips in the Slip Postings.
If there are any SIB postings in the Slip Postings then you can delete these by:
Highlighting the slip in Slip Postings on the Rejected tab.
Selecting the Delete button at the bottom of the screen.
If these are in General Postings do the below:
Highlight the SIB line on the right hand side of General Postings.
Select DELETE at the bottom of the screen.