I meant was there error code, some additional message, anything else beside the plain text you’ve mentioned.
The only difference between free and paid version is the footer link, so it doesn’t work on paid version too.
Is it working with default theme?
This seems like it’s on Bloglovin’s end. I tried couple blogs to add, with default theme, and I can’t add any site. Anyway, this kind of sites are looking for a feed url, and that url has nothing to do with any theme, like: https://www.simplewpthemes.com/feed/.