Since you installed a new OS you probably don’t have any more of your private and public keys that you used to sign your app in to XCode before. You need to regenerate those keys on your machine by revoking your previous certificate and asking for a new one on the iOS development portal. As part of the process you will be asked to generate a Certificate Signing Request which is where you seem to have a problem.
You will find all you need there which consists of (from the official doc):
1.Open Keychain Access on your Mac (located in Applications/Utilities).
2.Open Preferences and click Certificates. Make sure both Online Certificate Status Protocol and Certificate Revocation List are set to
3.Choose Keychain Access > Certificate Assistant > Request a Certificate From a Certificate Authority.
Note: If you have a private key selected when you do this, the CSR
won’t be accepted. Make sure no private key is selected. Enter your
user email address and common name. Use the same address and name as
you used to register in the iOS Developer Program. No CA Email Address
4.Select the options “Saved to disk” and “Let me specify key pair
information” and click Continue.
5.Specify a filename and click Save. (make sure to replace .certSigningRequest with .csr)
For the Key Size choose 2048 bits and for Algorithm choose RSA. Click
Continue and the Certificate Assistant creates a CSR and saves the
file to your specified location.