Can’t buy Shroud Of Cooperation / Cloak Of Coordination.
I am revered with my guild too 
even tried relogging / reloading, restarting my computer, nothing Is working.
There is a known bug with buying some items from the guild vendor at the moment. In the future I’d recommend you search these forums for other reports of bugs like this to find out if the issue has already been identified by others.
Also unable to buy multiple items I should have no issue purchasing from guild vendors. Error: “The Item was not found”