Go to your multipass vm
multipass shell foo
Edit ssh config and allow PasswordAuthentication
sudo nano /etc/ssh/sshd_config
find "PasswordAuthentication no" and make it "yes" instead of "no"
restart ssh
sudo systemctl restart ssh
go exit
exit
from your host do try ssh now.
ssh ubuntu@multipassIp (get ip from multipass list to show ip of your vm)
say yes if it asks for fingerprint, input your password.
And it shoud work by now.
if not try this
multipass shell foosudo passwd ubuntu
set your password again,
and exit try again to ssh.
And you could try this too.
inside multipass vm
sudo usermod -aG sudo ubuntu
exit and try ssh again.