STO P.O item deletion error

Hello Experts

I am trying to delete the intra-STO (UB Version) line item.

Experienced the below error, using Account asst cat: Q and item cat: U. 

Infact supplying plant is an another Plant ID and not using the Vendor number. Though, system throws irrelavant error ...

Any suggestion is appreciated.............

Document type YZ allows only vendors with plant assignment

Message no. 06804

Diagnosis

You have chosen a document type for stock transfers. Only vendors to whom a plant has been assigned can be used for this type of order.

Procedure

Choose a different document type or a different vendor.

Regards

MM