Distribution certificate / private key not installed

Question

Up to date (January 2021) (Xcode 10 - 12)

1. Go to Xcode - Preferences - Accounts - Manage Certificates
2. Click on the + at the bottom left, then Apple Distribution
3. Wait a little, then click Done

That's all. You may want to revoke the old certificate on developer.apple.com too.

Old answer

Step 1: Xcode -> Product -> Archives -> Click manage certificate

Step 2: Add iOS distribution

You can only have one distribution certificate. It unites a public key, known to Apple, with a private key, which lives in the keychain of some computer. If this distribution certificate was created on another computer, then the private key is on the keychain of that computer. And this distribution certificate does not work without it.

So to use this distribution certificate on this computer, you must find that computer, open Keychain Access, locate and export the private key, mail it or otherwise get it to this computer, and import it into the keychain of this computer.

If you go into the Accounts pref pane in Xcode and double-click your Team, you'll see a dialog that gives you help with this. If you see your distribution certificate and it says Not In Keychain, you can control-click that certificate to get a menu item that lets you email whoever created the certificate and ask them to send it to you. That person can use this same import to choose Export Certificate and can email you exported certificate.

Either way, the private key or exported certificate will be passworded. You'll need to know the password in order to use it.

This answer is for "One Man" Team to solve this problem quickly without reading through too many information about "Team"

Step 1) Go to web browser, open your developer account. Go to Certificates, Identifiers & Profiles. Select Certificates / Production. You will see the certificate that was missing private key listed there. Click Revoke. And follow the instructions to remove this certificate. Step 2) That's it! go back to Xcode to Validate you app. It will now ask you to generate a new certificate. Now you happily uploading your apps.

Add a new Production Certificate here, then download the .cer file and double click it to add it to Keychain.

All will be fine now, don't forget to restart Xcode!!!