If you wish to use Megascans exclusively with Unreal or Twinmotion, you need to accept the UE4 EULA.
To accept the EULA for Unreal Engine, first head over to epicgames.com and click on Unreal Engine> Get Started Now. You'll then need to select the type of license that would best suit your needs.
For the Creators license - the EULA will appear once you've clicked on Download Now.
For the Publisher License - you'll need to download Epic Launcher, then Unreal Engine. The EULA will appear as soon as you click "install" on UE, for the first time.
After you've accepted EULA for Unreal Engine you'll be able to subscribe via quixel.com/pricing by scrolling down to the Unreal banner and clicking "Get Started":
Please note that on this plan, you may use the assets in other tools, as long as the end product is in UE4.
Don't forget to check out the FAQs for this plan, found at https://help.quixel.com/hc/en-us/sections/360000977797-Unlimited-Access-for-Unreal-Engine.