It says 1 of 1 thumbnail images. I have no option to choose a thumbnail.
I was on the support forums in wordpress.org and it was suggested it could possibly be related to the theme, which is why I was inquiring here.
Have you tried sharing my sample post?
https://ronangelo.com/add-a-featured-image-on-a-wordpress-post/
When I try to share the url above it gives me a 1 out of 4 image option so it can’t be the theme. It might be the plugin you’re using.