Very common "doh!" You have copied the corresponding item in the keychain,
BUT!
you forgot the private key part:

So, the disclosure triangle on the left is missing, and part of the key symbol is missing what happened.
Solution: just EXTEND the triangle and select BOTH details, then click Export. move the file to another Mac and open.
ALSO - do not forget that in Xcode you must literally log in these days.
Xcode - Settings - second tab == Accounts

It's easy to forget, say, one of your various customer accounts.
Fatie
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