BT Cloud is only available if you've got BT Broadband. If you've only got a phone service from us, then you won't be able to get BT Cloud. Only the bill payer (main account holder) will have access to BT Cloud and so you might not be able to access it if another member of your household is the account holder and has activated the service.
If you've got multiple billing accounts with BT you'll need to select your account with broadband from the dropdown list to be able to view your Extras, including BT Cloud. You can only activate BT Cloud on one account.