title: Distribution certificate / private key not installed
body: <p>Using Xcode 9.1, after building an iOS app, I want to archive it and upload it to the appStore for beta-testing.
  But I get the following issue after clicking the button <code>Upload to the App Store...</code> and choosing <code>Automatically manage signing</code>:</p>
  <blockquote>
  <p>&quot;My Name&quot; has one iOS Distribution certificate but its private key is
  not installed. Contact the creator of this certificate to get a copy of the private key.</p>
  </blockquote>
  <p>I do not know why this &quot;private key is not installed&quot;, but the Distribution certificate may have been created on a different computer or something. In any case:</p>
  <p>What is the simplest way to retrieve the missing private key and install it? In order to make things work.</p>