Got it, thank you Eskimo for following up this issue for many days.

yes, sorry, but not working in my company's macbook pro...

And I just have airdropped this to my another old macbook air to have a try, this file has just added to the My Certificates tab in Keychain Access. So, it works in my personal macbook pro (the source), App Center for building app, my old macbook Air, but my company's macbook pro....

it prints: /path/to/my.p12: data

Yes, I’ve quitted and relaunched it. And also restarted the OS but no luck. Do you know if we can re-install the keychain access? I’ve tried to find it in App Store, but not found it...

yes, I've exported into .p12 file and double-clicked it in my problem macbook, and inputted the password without error, but the cert and key still not in My Certificate tab, but Certificates tab and Keys tab separately. And for this .p12 file, I can use it in MS App Center for building app successfully. So, the problem seems my keychain doesn't work well....

BTW, I’ve tried to generate a CSR and uploaded to developer portal and got the cert and key installed. And also I’ve tried to copy cert and key by exporting from my personal MacBook( the one can go thru the code signing process) in one-go and imported to keychain access of my company MacBook(the one that code signing not working , as well as tried to export the developer account from Xcode from my personal MacBook and import to my company MacBook. I know that I need to prepare the cert and key together, but have no ways to get certs and keys in pairs and to show in my certificates tab... sigh...

So, is this problem resolvable? Could you advise what I should do next? Thank you.

Thank you for your reply! The result of the command is: 0 valid identities found

Have you resolved this? Could you advise? Thanks