Receiving error on Purchase Requisition after changing tax rate on existing tax code

We changed our tax rate on an existing tax code from 10% to 0%. The change seems to be working in Receivables as well as on AP invoices and Purchase Orders however, when creating a Purchase Requisition using the tax code (with changed rate) we get the following error:

'System cannot determine the default tax rate for tax classification and status AU GST STANDARD'

Is anyone familiar with this issue and why it only seems to affect Purchase Requisitions (not other areas)?!

Thanks in advance -

Karen E.