I've followed the guide here: https://www.linode.com/docs/tools-reference/tools/limiting-access-with-sftp-jails-on-debian-and-ubuntu, but I'm getting 'Authentication failed' when the user connects via sftp, and after the user has been added to the system group filetransfer.

I'd greatly appreciate some assistance here.

Thank you.

Did you add any new usernames to 'AllowUsers' under the '# Authentication:' section of the 'sshd_config' file?

e.g.

# Authentication:
LoginGraceTime 120
PermitRootLogin yes
StrictModes yes
AllowUsers root username username username

It's something I regularly forget to do when adding new sftp users, and wonder why they can't log in.