You probably have an empty instance in your settings (eg. lemmy.ml but with no account under it) and were browsing the “Everything / All” feed, you went inside a post served “via lemmy.ml” and tried to upvote. Obviously it asks you to login in that case.

You can remove instances with no accounts by long pressing them. Also you can hide the everything feed from settings ;)

Also, Liftoff requires you to login to each individual instance to upvote anything from that instance for some reason.

Uh, I just upvoted you and this post from feddit.it using Liftoff