App can not be installed via TestFlight: Registered maximum number of devices

There is a maximum number of 10 devices that can be registered to each user.

To remove a device from your user account:
1. Launch TestFlight on one of the devices you are signed in.
2. Tap on the "Apple ID" button at the bottom.
3. On the next screen click on the "Edit" button on the top right.
4. Select a device in the "Other Devices" section.
5. Click on the "Remove(1)" button on the top left.

Note: You cannot remove more than 5 devices per week.