Hello Abdessamad EL-BAHY,
Please check whether you have defined the transaction type for PO in SRM. If you don't get the switch in SPRO. Please look into this link
Defining Transaction types in SRM - Supplier Relationship Management - SCN Wiki
Also please do check in backend that whether you have maintained the document type for PO in ECC.
By standard ECPO would be there for SRM. But if you are in Classic Scenario then NB would work fine for you.
Gaurav Gautam