Assign Sales Unit to Sales Order

Hi,

I have found the Party with a query try to assign it to the Sales Order. I have read in Repository Explorer that a SalesUnitParty node have a public write access, but when I try to assign the Party to this node it isn't possible: the node is red underlined in Studio and the message is showned:  "SalesUnitParty is read only". What am I doing wrong?

Regards,

Boguslaw