As the title reads, when I try to launch UE4 4.20.1, it instantly gives me an error saying that the prereq’s failed to install. However, I’m still able to launch UE4Editor.exe manually as well as projects in the My Projects section of the launcher.
Double check the c++ side of your project. If you don’t have a c++ side active, I would just have the EGL run a verification on the install, in case you haven’t yet.