2nd this. Not too complicated but also feature rich. Transmission is good too but a lot of tweaks are only configurable with file changes followed by a restart. This is a big PITA for someone like me that uses Docker.
Also, the old Transmission version (3.0.something) that is in either Ubuntu 18.04 or 20.04 has a memory leak. It gets killed every week or so in one of my Ubuntu LXCs that only has Transmission and OpenVPN with 4gb ram available.
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.
I've always been a fan of Deluge. The Thin Client interface is the bee's knees. But, QBittorrent is my new fave. Wish they had a thin client instead of web though...
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.
I usually make a dedicated user for torrent clients. Works well. This allows me to run multiple instances of transmission in the vm, for example. Manage them through systemd services.