Ubuntu 20.04 "NO LanMan password set for user"
I think you are almost there.
Your HP printer is trying to connect to the Ubuntu server using the SMB1 ( Samba calls it NT1 ) dialect of smb. In Ubuntu 16 or 18 that wasn't a problem. In Ubuntu 20 Samba turned it off by default.
You can enable it in Ubuntu 20 by editing smb.conf and under the workgroup = WORKGROUP line add this one:
server min protocol = NT1
You can try to restart smbd sudo service smbd restart but you may have to just reboot.