Linked Documents not working in builtin doctype

i want to create new linked document in purchase order .
when i want to add it , it’s disable and i can not add any items.

i can do that in new doctype that i created
what should i do

It seems a permission problem. How is your doctype look like?

i am a system manager and it seems i have full permission on purchase order

I mean the new doctype may have a permission problem. You have created a child doctype I think. Check its permission.

new doctype is ok . i have a problem to create linked document in purchase order