When you play the game, you're probably viewing in in "Free Aspect". The UI in Unity is a bit tricky, but go to your canvas and look for the box that says "Constant Pixel Size", then change that to "Scale with Screen Size". Then just try to position the button wherever you want and it should work.
(Sorry to bother you again) So your solution worked, I consider the game ready for publishing but I’m running into another error. My WebGL build only works on ‘build and run’, and it does nothing when I try to open the HTML file or put it on Itch. Everything I’ve seen online thus far doesn’t seem to have the answer to my dilemma. Any ideas? Edit: I’ve managed to make it work by putting the link (localhost:[numbers]) from the build-and-run but the name of the URL makes me think the page isn’t going to work on anyone else’s computer.