Sometimes you submitted app on a system and try to submit the updated verrion of the same app from different system (Macbook,iMac), That creates a problem because there is not a private key on your Mac associated with the distribution certificate you are trying to use to sign the app.
If the computer who requested the distribution certificate is available (or there is a backup of the distribution assets somewhere)
From the computer where the distribution asset was generated, open Xcode.
Click on the Preference menu from Top left Xcode Menu
Click on the Accounts
Click on the User Agent from Accounts
Then Click on the View Details
Then Select the Profile(Distribution or Development) you want to Export
Then Click on the ‘+’ Plus button and select you preference option to export
Save the File on Desktop.
Double click or open the file on the keychain.