I’ve currently got my site set up on Wix

That’s the problem. If your domain (let’s say example.com) is currently directed to Wix, then example.com/wp-admin/ won’t load because that /wp-admin/ bit doesn’t exist over at Wix.

You’ll need to either direct your domain name to Site5, start up a new domain name at Site5 and reinstall WordPress on that, or contact Site5 support to see if they have any temporary domain options for you.

I’ll have another google of How to transfer Wix to WordPress and ask site5 too.