If you see the error below for Quickbooks users.
"A business validation error has occurred while processing your request:
Business Validation Error: Select a product or service"
This means the account being used on the bill has likely been deleted in Quickbooks.
They need to check if their chart of account in M.D matches their chart of accounts in Quickbooks.
They may need to open the bill in mechanicdesk and change the expense account its linked to.
Then do a quick sync