Unable to use key file “id_rsa” (OpenSSH SSH-2 private key (old PEM format))
Background information , taken from the PuTTYgen Help section ::
8.2.12 Dealing with private keys in other formats
Most SSH-1 clients use a standard format for storing private keys on disk. PuTTY uses this format as well; so if you have generated an SSH-1 private key using OpenSSH or ssh.com’s client, you can use it with PuTTY, and vice versa.
However, SSH-2 private keys have no standard format. OpenSSH and ssh.com have different formats, and PuTTY’s is different again. So a key generated with one client cannot immediately be used with another.
Using the ‘Import’ command from the ‘Conversions’ menu, PuTTYgen can load SSH-2 private keys in OpenSSH’s format and ssh.com’s format. Once you have loaded one of these key types, you can then save it back out as a PuTTY-format key (*.PPK) so that you can use it with the PuTTY suite. The passphrase will be unchanged by this process (unless you deliberately change it). You may want to change the key comment before you save the key, since OpenSSH’s SSH-2 key format contains no space for a comment and ssh.com’s default comment format is long and verbose.
This error shows up while using Putty to connect to a MikroTik´s CHR or CCR or Routerboard as Putty has and needs its own Private Key File format.
Simple solution as stated in PuTTYgen Help section is to load the Private Key into PuTTYgen and as seen in PuTTYgen itself , this Key will be converted into the Putty Private Key format. There you can change the “Key Comment:” as PuTTYgen will automatically fill this with “imported-openssh-key”.
Save this Private key again and now you should be able to login with Putty to a MikroTik box with Key file authentication.