I got stuck in this problem for like 2 days now. I have updated my OS and Xcode so I have to download my developer certificate. When I accessed it in the keychain access it doesn't have its private key. And Xcode throws an exception that private key is missing. I have done revoking and requesting the certificate but it still does not solve the problem. What solution can possibly solve my problem? Thanks.
Private key is needed, it is fundamental. Try to find it. If it is really gone, Use a new private key to apply a new certificate.