If you get “SSH handshake failed” when trying to use Guacamole to connect to Ubuntu via SSH, you need to use a workaround:

In the meantime a workaround is adding “HostKeyAlgorithms +ssh-rsa” to the end of /etc/ssh/sshd_config on the Ubuntu machine and restart sshd. Note: I don’t have an understanding of the security implications of this, so use at your own risk

The solution was posted here:
https://www.reddit.com/r/linuxquestions/comments/ued2vq/comment/i736why/?utm_source=share&utm_medium=web2x&context=3