I have no idea what that white ring is or how I applied it. Here's a picture of it in edit mode: I want to get rid of it but I don't know how to go about doing so. I'd appreciate some help. If needed, here is the file: https://pasteall.org/blend/07f0ab12a84944e69b2afd50cf7d3f6f
The issue is being caused by the presence of an extra face inside the mug. Its a quick fix; in edit mode, select it and press X > Faces.
Also note that there's an Ngon near the base of the mug that could use some splitting, for uniformity's sake. In edit mode, select the top and bottom vertices and press J .
This should do it, good luck!
Ok I found the problem. There is a face on that ring, and you have to delete it. I thing you accidentally made a face there and that is why you can't chose the whole ring if you select it with Alt + LMB
I am not sure if I made it clear, but look for a face which if shouldn't be there.