Have you checked the following?
- There is a copy of the public key, i.e. contents of file id_rsa.pub, in the <user_home>/.ssh/d_rsa.pub on the target machine.
- There were no extra characters such as blank lines or spaces inserted when you cut and pasted the private and public keys into the fields in the Credential Properties section.
- If there are /var/log/secure.* file on the target machine, look at them for clues.
If necessary, see the following doc for information on setting ssh key named credentials.