Now my problem is how can I delete the certificate I have created ( vpn only clients) instead of just revoking them.
It's not possible to completely delete a certificate (well, it can be done manually after being revoked if you edit the file /opt/phpki/phpki-store/CA/index.txt, but it's dangerous as you can corrupt your certificate database). The question is why would you want to delete a certificate instead of just revoking it ?
And also when I try to renew the certs but it give me an error that say's "This was likely caused by entering the wrong certificate password." what password did it requires? I want to renew the certificate because I forgot the password of the connection of this certificate.
Renewing a certificate will use the same CSR, and the same private key as the old one (so the same password). This is an issue with PHPki, as if a private key is compromised, a new private key should be generated if the certificate is renewed. For now, I suggest you just let the old one revoked, and just issue a new certificate with a different common name.
Regards, Daniel