Signing in on one of the other mentioned devices, may help complete what is missing, or notify what is wrong in more detail, or confirm what needs to be confirmed. Depending on the issue, just signing in once may be enough.
Did you complete all the requested user details, payment method (+expire date +security code), and confirm the email address for a new account (respond to test message)?
Before you can sign in on the web …
On the TV, do not use the web interface, but instead use the Apple TV app or Apple TV+ app. The Apple TV+ app may need to be used for redeeming the LG offer. Sign in through the in-app Settings (sprockets wheel icon), with the Apple ID account as used for the offer.
How to create a new Apple ID - Apple Support
↳ If you see a message that you need to sign in to iTunes
Get 3 months of Apple TV+ free - LG Support
Redeem as instructed, or the offer may not be applied.