Whoever told you that is wrong. I have a build pipeline that I set up two years ago that uses butler for uploading games, and it works just fine. It always has just worked fine. Of course, it will break occasionally if itch.io itself has an outage, in which case you should retry later.
Because of that, it is not possible to fix your problem without the exact error message. butler uses HTTP and can run into every potential failure state that HTTP can run into, and I am not giving you an extended lecture on how HTTP works. Show me an error message, then I can try and help.
(Probably better to do that in its own thread than to bury it here, though.)