V10 - Buy TO order Overwrite

I user Epicor 10.0.700.2.  I add a sales kit on my sales order entry.  In my componets kit, I checked Buy to Order.  It's ok.  But if not possible to change the supplier, I don't know why ?  The supplier is gray.  On the section Buy to order I have the message, PO Not linked, override.


Why buy to order the supplier is not accessible ?


Thanks Nancy