In case someone accidentally encrypts a server key (e.g. not following directions) then expects it to be accepted in to Cpanel, you’ll need to decrypt it first.
Most web hosting platforms (like Cpanel) need the server key to be in clear text. The private key can be decrypted with:
openssl rsa -in encrypted.pem -out plaintext.key
Make sure whomever encrypted the key provides you with the pass phrase.