This may be for several reasons. Please check the following items to try and locate the reason for receiving the error message:
Are there any transactions in the Purchase Ledger batch for this Supplier?
Are there any transactions in the Purchase Order batch for this Supplier?
Are there any outstanding orders for this Supplier?
If you are still struggling to work out the cause of the error, to resolve the issue Support will need to run some scripts on the database. Therefore please raise a new case online and reference the title of this article and please include details of the supplier needing to be made inactive.