If you are on any kind of non-standard package, or have special discounts applied to your account, you cannot upgrade online.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV+), Talk Unlim Telco, VIVID 100, Virgin PAYG Mobile
Welcome to the community and thanks for posting. I'm very sorry to hear you've been unable to upgrade the Sky HD channels via your online account.
I've managed to locate your account to check this. I can see you've had the HD channels added since you posted. I'm afraid I'm not sure why you were getting the error message.
I hope everything is okay for you now. If you have any further issues then please let me know.