Xcode 8 introduced managed code signing, which automatically generates the App IDs, signing certificates, and provisioning profiles you’d usually need to set up manually for your app. This covers most aspects of getting your app ready for distribution, but one thing that still isn’t automatic is Xcode’s handling of devices UUIDs manually added in the Developer Portal; when new device UUIDs are added you still need to generate a fresh provisioning profile which includes them.

Luckily if you’re using managed code signing, all you need to do is delete your managed provisioning profile and let Xcode repair it. It will automatically create a new profile which includes any new devices you have added in the portal.

More detailed instructions on how to do this are here:

1. Browse to developer.apple.com, sign in, and add the UUIDs for your new devices
2. In Xcode, open up Preferences > Accounts, and select your Apple ID and Team, then click “View Details”
3. Right click and select “Move to Trash” on the provisioning profile matching the one automatically generated by Xcode (its name should look like “iOS Team Provisioning Profile: [your app identifier]”)
4. Shut down and restart Xcode
5. Browse to your project’s general settings - you should see your Provisioning Profile rebuilding
6. Once the provisioning profile has been rebuilt, your new devices should be included in it!
7. You can verify that your new UUIDs have definitely been included in the Provisioning Profile by clicking on the info icon next to the Provisioning Profile’s name, and dragging the icon of the Provisioning Profile onto your desktop. This will copy the .mobileprovision file to your desktop, which you can then open using a text editor. Look for the <ProvisionedDevices> key, and you should see the UUIDs you added earlier to the developer portal included in its <Values> array.
8. Rebuild/redistribute your app, and you’re good to go!