How to remove Organization Assignment from a Master Item

 Hi,

I have a Master Item that i've created that is assigned to Master Organization and to a child organization of Inventory.

Whenever i try to remove the assigment by uncheck the "Assigned" checkbox of the Inventory Org at Organization Assignment, i get the message "FRM-40200: Field is protected against update."

I've tried to do this both in Master and Inventory Organizations.

Can you please help me understand what am i doing wrong? Is there any Setup that forbids me of doing this?

Thanks a lot for your help.