Purchase requisition can be converted to purchase order.
But prerequisite is that the External Procurement Relationship has to be created and CIFed from ECC to APO.
There are three types of External Procurement Relationships: Purchasing info records, Contracts and Schedule Agreements. They can be checked in /SAPAPO/PWBSRC1.
The conversion can be Mass conversion /SAPAPO/RRP7, or it can be converted individually by setup the Conversion Indicator and double click the order in Product view - /SAPAPO/RRP3.
Check the transfer setting in /SAPAPO/SDP110, if it's the periodic transfer, then you have to use the Process Change Pointer /SAPAPO/C5 to manually transfer to ERP system.