On user profile details page you can enter your Youtube channel. The address path of the channel (/user) is outdated - Youtube no longer supports that kind of path. /USER should be replaced with /CHANNEL
Maybe newer channels have the /channel, but the first channel I opened still has the /user so I don't see the real issue here.
The real fix should probably be that there's no prefix, so you would manually add channel/user, whichever applies to your channel.
I've moved the "user/" part into the user editable field so that you can now include "channel/" in your YouTube name. Not really ideal, but it needs to be this way for now as old accounts with "user/namehere" may not work with "channel/namehere" so it's not a drop in replacement unfortunately.