Jenkins plugin ssh-agent showing "ERROR: Failed to run ssh-add"
This is just a wild guess since this could have different reasons.
But I just had this error, and my problem was that when I created the credential in Jenkins I forgot to add a linebreak at the end of the input.
......<all the usual content>......-----END OPENSSH PRIVATE KEY-----<linebreak needed here>
I got this error when I put the SSH public key into the Jenkins credential instead of properly putting the private key.
Note that the private key is the long one that looks like this:
-----BEGIN RSA PRIVATE KEY-----... lots of unreadable random characters for many lines ...-----END RSA PRIVATE KEY-----