Try creating a many-many relationship and you will get to see the one level depth attribute while selecting the display attribute from Multi-select widget. Find the below images for your reference.
Hope it helps you.
Have you tried this widget? It seems to fit your case, and has platform support!
Regarding your error: I think you should select your ‘Data source’ entity over at least one association, starting from your context entity.
So i wanted solution in 2 ways:
1. Existing thing should work as a Checkbox where i am able to select multiple answers but i was only able to single select.
2. or else I will use a ready made custom widget but i was getting error there.
My Solution :
Existing checkbox was allowing me to select only single answer So what i did was i enabled Editable on my list view and then i disabled Editable property on description and turn on Editable on checkbox and now i am able to multi select.