By the look of their Twitter timeline the launch seems to be a bit of a disaster...
I spent about 15 minutes buying, downloading and installing Arq 6.
Then, i spent about 10 hours trying to fix things, and eventually downgrade back to Arq 5 from another backup made with borgbackup...
https://twitter.com/arqbackup/status/1247845283326803968?s=2...
Also, the documentation has been offline all (launch)day: https://www.arqbackup.com/documentation/arq6
It's a shame because i've been hapily using Arq4 and 5 for many years. Hopefully they get their shit sorted quickly.