Hello,
it happened to me yesterday and i couldn’t fix it with all the solutions in AnswerHub. But i happened to download it anyways.
I had one user with email i am not using much and i didn’t find how to change it, so i created a new one and when i logged in Epic Games Launcher i got that message while trying to download the engine.
I tried all the fixes i could find but it didn’t help. So i logged in with my old user and everything worked as it should.
I hope it will help to find the fix. 
Also feature to add/change email in your account would be nice