SSH supports two different and incompatible protocols: SSH1 and SSH2. SSH1 was the original protocol and was subject to security issues. SSH2 is more advanced and secure. Rationale: SSH v1 suffers from insecurities that do not affect SSH v2.
Solution
Edit the /etc/ssh/sshd_config file to set the parameter as follows: Protocol 2 Restart sshd : systemctl restart sshd