Hi,
First of all, check the vendor master purchasing view, whether any schema group is given. Now note down your purchase organization given in the PO.
Now check the schema groups for your purchase organization and vendor in the path SPRO-MM-Purchasing-COnditions-Define Price Determiantion Process-Define Schema Determination-Define Schema Group.
Check your price determination process in the path SPRO-MM-Purchasing-COnditions-Define Price Determiantion Process-Define Schema Determination-Determine Calculation Schema for standard purchase orders.
Check whether any pricing procedure has been assigned to your purchase organization and vendor schema group.
This will solve your issue.
Additionally, you may check the authorization also in SU53 from PO screen, if its a new user id, since missing authorization objects may hide the condition tab.
Hope its helpful.
Regards
AKPT