So, the reason it's likely erroring for you is that pop ups are blocked. The program I use has the achievements come up as pop-ups, so when your browser blocks them, it will also be blocking some of the code from playing out, which then results in an error. 

I've seen itch.io marking downloads so I figure there has to be some way to, but I'm not quite sure how if there is... I'm very new to uploading here, I'm sorry to say. 

If unblocking pop-ups doesn't fix it, the only thing I can think of is either trying it on Dashingdon or Moody.ink, as firefox should run it fine otherwise.