Ubuntu Server 20.04 SSH multiple public keys in authorized_keys not working
 

As a beginner i create a secure ssl private/public key login for root.
I understand this is bad practice and created a new user and added their key to the file authorized_keys which now has root and new user key. When I try to login via putty with the private key it does not allow me. What is the process to do this. I assumed as the file says authorized_keys it means multiple keys can be added, or am i wrong?

SSL / TLS are different than SSH. I presume you mean SSH.

If you created the key pair using PuTTY, you'll need to convert the public key to the format used by OpenSSH before copying it to the server.