Cannot sign the App in XCode, says Missing Private Key.
When you create and install your your Apple WWDR certificate but submitting the CSR (Certificate Signning Request) via KeyChain, it installs the public and private key of your cert into the Keychain. In this case, some how your private key is not avaiable/missing from the Keychain. This generally happens if you are trying to sign the app via XCode on new computer. Since the keychain in this computer does not have the cert private key, XCode fails to sign the app.
Go to keychain in the old computer where the original cert was generated and export the private key and then import it back into the new computer.
If you don't have access to old computer or somehow cannot get the key, you can invalidate the current certificate and create new cert from this computer by submitting the CSR via keychain and then installing the cert.