The error is of concern. Describe more about how you added the Item. Through the UI? Independently or did you click through from the Channel to create the Link and then from the Link to create the Item?
Walk us through the steps you are taking in more detail. Unfortunately there are lots of ways to do this and it’s not at all clear which one you are using.
Why don’t you try to start from the Channel in order to link (that is the term used for “adding” a channel to an item).
Select the thing, on the the thing select the channel to be linked, click to green “+” sign ( “Add Link to Item…”), don’t change the selected “Use an existing item” and expand the item list in order to select your altready created item. At last click to blue “Link” button.
Because I haven’t got a clue… (no kidding; I wonder at times how many v1/2 users are out there who never made the switch to the ‘thing’ area and struggle with it all)
It did not occur to me… I did’t see the Channels tab. Was quite surprised what’s all in there.
I have now linked it… but didn’t see the green plus, until I used the ‘right arrow’