Lots of variables, I just happened to see this post but I thought I could at least help you get started troubleshooting.
What device are you using? Do you get the same error if you're on a different device(Android/iOS vs desktop/laptop)
What browser or app are you using(Safari, Chrome, Firefox, Jerboa, Mlem?) If you can use a browser and open dev tools(F12) and take note of what happens(warnings/errors POST requests) when clicking on the upvote button may help the devs of the app(if you're using one) or instance maintainer figure out what's going on
Have you checked to see if the post you're trying to upvote is within your instance vs a federated post?
I'll reply if I can later, just thought I'd ask these questions to get you started. May want to check > out !/c/techsupport and/or !/c/lemmy_support to see if there are people having similar issues.