Login to your server as root.
As the root user, edit the sshd_config file found in /etc/ssh/sshd_config:

vim /etc/ssh/sshd_config

Add the following line to the file, you can add it anywhere but it’s good practice to find the block about authentication and add it there.

PermitRootLogin yes

Save and exit the file.
Restart the SSH server:

systemctl restart sshd
service sshd restart