Message : Document type doesnot allow you to assign to object VBAP

Hi All,

I am attaching a document that is been created through CV01N to transaction VA02 at item level.when i am trying to put that document no. at item level on the Extras-Documents- system is giving me a message as "Document type doesnot allow you to assign to object VBAP".

similarly if i take this document no from F4 screen,it comes but when i check the same after saving the document gets disappeared.

while the same document is getting attached at material master easily.

I think some customization is missing for the same..plz let me what i need to do to resolve the same.

Thanks & Regards,

Sandeep Sharma