When this error appears, the system has identified that the account is still in use somewhere. Check the following to identify the cause:
Is the account being used as a head office account?
Are there any transactions in the Sales Ledger batch for this customer?
Are there any transactions in the Sales Order batch for this customer?
Are there any transactions in the Sales Order batch for another customer, but delivering to this one?
Are there any outstanding purchase orders with this customer as the delivery point?
Are there any active project codes with this customer set against them?