Try the below basic troubleshooting steps:
Update to the latest version of iOS or watchOS.
Confirm that you have Face ID, Touch ID, or a passcode set on your device.
Check that your Apple Account is signed into iCloud with two-factor authentication turned on.
Check whether Apple T&C's were accepted in your settings when logging into iCloud.
If you’re still unable to add your room key contact our Central Reservations Team on 020 7278 7871 or info@imperialhotels.co.uk