Do the following commands:
sudo apt-get update
sudo apt-get install openssh-server
Edit the following file:
/etc/ssh/sshd_config
Make sure it says this in the file
PermitRootLogin yes
Restart SSH:
service ssh restart
Reference: https://linuxconfig.org/enable-ssh-root-login-on-debian-linux-server
https://linuxconfig.org/enable-ssh-root-login-on-debian-linux-server