The first time you install a UE4 version (e.g., 4.26, 4.27) from the Epic Games Launcher, the launcher will automatically trigger the prerequisites setup. It runs before the engine binaries are verified.
Open regedit and navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\14.0\VC\Runtimes\x64
The Installed DWORD should be 1.
A small window titled "UE4 Prerequisites -x64- Setup" appears, displaying the message: "Setting up Unreal Engine 4 prerequisites. This may take a few minutes." Ue4 Prerequisites -x64- Setup