Pro tip: when copying a link to a Lemmy/Piefed/Kbin/etc post, please make sure you use your instance's option to copy the link to the post on its original instance (the instance the post was made on), rather than simply copying the link to the post on your instance out of your URL bar. On your instance's web interface, that option is in the More drop down next to the Boost button, just below the post title. (On a Lemmy instance, the link to the original post is the Fediverse icon right next to the upvote and downvote buttons, and you can right click and Copy Link from there).

Obviously, when you open a post in the browser on an instance that isn't yours by clicking a link (for example, this link to this same post on my instance, which is a vanilla Lemmy instance), you can't interact with it, because you aren't logged in, because you don't have an account on that instance. To interact with it, you need to open the same post through your own instance.

If you have a link to a post on the instance it was created on, you can simply paste that URL (the whole thing) into your instance's search bar to open it on there, so you can vote/comment/etc. Some mobile apps can even do this automatically. However, this doesn't work if you paste a link to a post through an instance other than the instance it was created on. In order to interact with the post you just linked to, people have to click the link to open the post on your instance, then click the button on there to copy the original link, before pasting it into their own instance. It's polite not to make them go through that extra work.

Judging by your account age, it seems you're new to Lemmy, so you probably didn't know that. Just something to keep in mind for future. :)