Aws key pair puttygen
AWS EC2: generating private key file out of cert-***.pem for SSH terminal access
The ssh private key is completely different from the X.509 certificate and private key. It's a little confusing because folks often save the ssh private key generated by EC2 in a ".pem" file just like the cert and pk use.
You can't convert or use the X.509 certificate or private key for ssh as you tried to do.
You can generate an ssh private key (sometimes called a "keypair") through Amazon EC2 on the AWS console or through the AWS command line tools (ec2-add-keypair). If you are using Putty, you may still need to convert to PPK format as your referenced article describes, but you are converting the ssh key .pem file, not the X.509 private key or certificate.
