Your account already has a signing certificate for this machine but it is not present in your keychain. To create a new one, you must first revoke the existing certificate.

Same problem for me after I create a network extension. My Company has a capacity to develop network extension. But me as a developer of my Company do not have a capacity to develop it. This is really a nonsense.

This drove me crazy - going around in circles, revoking certificates, downloading random certificates etc etc. It turns out it was, as another poster has said, my system date and time wasn't set as automatic when it should be, so when I was revoking and installing new certificates, the times didn't match exactly and the certificates weren't showing as valid in Xcode. I deleted the current development certificate in my keychain, reset my time and date to be automatically set from the apple server. restarted my machine, checked the time was correct, opened Xcode, revoked the current certificate to force a new one to download, then it all seemed to work.

the reason my time wasn't set to automatic was there's a bug in date and time which causes the location to be incorrect (I am in London but it comes up as Berlin as my location, so I had manually set my time.)