You might be chasing two different problems. I had the UI Action error in the console too, except my item was working. Not knowing what was broken, I went around and around until I found this article:
ServiceNow states this is a known error and will not be fixed. Perhaps in your case, this is harmless as well.
I don't know what troubleshooting you have done, but where I would start is:
a) Confirm I am using an OOB widget (not a customized one).
b) Create a MVRS with a single line input question and try it out (no policies, no anything) on a new catalog item, a new page, and using the OOB widget.
My assumption is that something is not right, and by creating a simple MVRS on a new item, a new page, and using the OOB widget, we should be able to demonstrate things are working. Then we can focus on your specific item. For example, your item may have client scripts not running on Service Portal:
If we can get a basic MVRS working, then it's easy to figure out which custom piece of your item is not working.
If you are still not able to have a MVRS submit when using the OOB widget on a brand new page, it seems like something is really wrong. This should always work, and if not I would immediately escalate a ticket to HI because that is using all OOB items.
