The reason is your TV have webos 3.0 and for some reason hbomax requires webos 3.5 which is absurd indeed. Netflix, Prime and many others have built apps that works on 3.0 or earlier.
Basically webos 3.5 came with LG TVs built 2017. C6 and my own B6 is built 2016. The base webos can not be upgraded, you are stuck on 3.0.
If it is a hbo or LG decision to require 3.5 for hbomax app I don't know. It locks out all LG TV owners with a TV bought before 2017. It is not like you change TV that often, every 4 years.
I ended up buying a Chromecast with google tv. It is not optimal and I really hope LG and hbo will release a hbomax app supporting webos 3.0.
