We believe you are getting this error b'coz of funds check.. A fund checking determines if there are funds available for the requisition based on the applicable budget and if the cost is allowable. If funds are not available for the requisition, the requester receives immediate notification that funds are not available and the requisition may not be processed.
Could you please check Profiles, 1) POR: Enable Check Funds, is it set as "Checkfunds" ??
2) PO: Override Funds Reservation