I have same issue. Very annoying. I have urgent tasks to complete, now unable to finish them.
Any fix? I've got same issues, spent a day but nothing.
I have figured it out... Since many years, I have attached a smartcard to my computer which contains a certified private key to sign emails and PDFs. This never was a problem when building apps with Xcode. Since I have installed 10.14, I get the "The specified item could not be found in the keychain." error when I have attached the smartcard. The solution in my case is to simply remove it.
More generally: When the smartcard is inserted, I see an additional keychain in the Keychain Access app. It's the first one in the list and it is locked. My guess is that Xcode / codesign only tries to find the code signing certificate / private key in this additional keychain instead of in the login keychain. So the general solution may be to have a look at Keychain Access and check if there is anything unusual.