I had the same issue for weeks now. I kept on trying week after week until I got frustrated and decide to look up a solution online. I found this thread and was able to resolve the issue in less than a minute. Here is what I did.
1. I didn't bother to downgrade Impactor
2. On the Impactor Tab, Select "Debug Provisioning..." (Not that I know what that means or does)
3. On the Xcode Tab, Select "Revoke Certificates"
4. Entered my Apple login details and got a message that says 2 certificates were revoked or so
5. Then I proceeded to sideload Electra and it went smoothly without any hitch.