qBittorrent?

Not the nost popular choice but I’m happy with Transmission. I figured out a a way to make my weird VPN subscription work with Haugene’s docker image years back and it’s working nicely for me.

qBittorrent tends to be the popular one but all the standard FOSS clients should work fine with Sonarr (e.g. qBittorrent, Deluge, Transmission).

I don’t have Transmission in front of me here to give you specific guidance but auth failed basically means you should double-check Transmission’s web ui user/password/port number & make sure it’s the same both there & in Sonarr. The same would apply for any other torrent client you want to set up in Sonarr.