Why was PC-A unable to access R1 via ssh?

It could be that an ssh virtual terminal has not been configured for the port connected to PC-A

Explanation:

The computer network is an interconnection and intercommunication of computer devices to share resources. Intermediate devices like the Router and Switches are used to create physical and virtual connections between networks and nodes.

virtual connections like the default Telnet and the and more secure SSH are used to remotely communicate with intermediate devices for management purposes. For example, to use the ssh, the line vty and the specified virtual port range must be configured with a username, a password, an encryption key and other ssh features before a connection can be made.