You will need to be subscribed to the Unreal Unlimited subscription plan offered by Quixel via quixel.com/pricing.
If you are not subscribed to the Unreal Unlimited plan, you will get the following pop-up when trying to access Megascans within Twinmotion:
You will be required to link your legacy Quixel account created before November 2019 to your Epic Games account, if required. To link an existing account, simply sign in with your Epic Games account via the "Sign in with Epic Games" option, and you will be faced with the following:
In case of any issues, please feel free to write in to Quixel Support at email@example.com.