I was having a problem with codesigning my apps, so I deleted all the keys from the keychain. Then I went to Certificate Assistant => Request a certificate from a Certif
Below link resolved the issue for me. https://forums.developer.apple.com/thread/72863
In my case it was the yubikey which was connected to my Mac all the time. After removing it, issue got resolved automatically.
Choosing the private key in keychain will occur the problem. "the specified item could not be found in the keychain" While you choose the public key solve it. "Your certificate request has been created on disk."
Okay the name that shows displays the item you highlighted in the Keychain mainwindow (if you select a different item here you also see the menu item change with it). If you go the the category section on the left and select my certificates for instance and go back to the certificate assistent menu item it looks/acts like you're used to. Hope this helps!
After revoking every certificate I had, I figured out the problem was with my yubikey. It was getting in the way of signing the app. After I removed it, problem solved.
This forum thread helped me out: https://forums.developer.apple.com/thread/106938
Solved it! Had to change "User certificate" to "Code signing".
I solved it. Ensure you are in the "Certificates" section and you select "Apple Worldwide Developer Relations Certification Authority" before requesting a certificate.