Hi Amit,
You get this error if you have not configure the widget correctly. Please can you update your question and add more details with some screenshots if possible about the issue?
Below is the screenshot of error
Hi Amit,
The following forum answer will help you to fix this issue
Hi Armit,
Try creatie a many-many relationship and you will get to see attribute from Multi-select widget. Find the below images for your reference.
Hope this will help for you
Thanks,