Why i can't transmog this item

Boundless gaze anyone knows why i can't transmog this item i can wear it also.
https://prnt.sc/fCDFF7PHy6LV

You can't transmog Mail into Cloth, maybe you are trying that.

I can't even extract essence from item

Unequip your boots, then try it. Yeah, I'm serious..

You are a hero lol !!!

For those who are wondering:

http://wotlk.cavernoftime.com/item=49110 is unique equip. So he couldn't equip the linked head, as his boots has http://wotlk.cavernoftime.com/item=49110 in a socket.