exchanges a key to play paragon and when I open the launcher keeps asking me to buy it HELP PLS¡¡
You’d be better off posting this in the Paragon forums, rather than the marketplace for Unreal Engine 4 forums 
even better, contacting support.
Link to Paragon Player Support
Not trying to be an asshat or anything, but the people that frequent this forum aren’t really going to be able to help you much is all 