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.
If I do not cut and paste please suggest an alternative to upload the contents id_rsa and id_rsa.pub directly to the Named Credentials page.
Instead of cutting ans pasting, you can upload the keys using the Upload Private Key and Upload Public Key options as documented in section "188.8.131.52.2 Setup Example Session" of the doc that I referenced earlier.