We have same problem, it’s because UnrealVersionSelector is broken now:
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same issue 
I’ve got two UE 4.27.2 projects – one built with Blueprints and the other in C++. The issue only occurs when I try to open the Blueprint project. The C++ project launches without any errors.
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Same problem here! Unreal fails to generate project files.
Luckily you can bypass this all using a couple of direct commands, but come on
Yup, this happened to me last month and is still occurring. However, I knew it was likely to happen because I use Windows 10. When they announced the end of Win10 support, I was expecting all sorts of weird things to happen with the Epic Launcher (I have no idea if it’s intentional or just “an expected” coincidence).
The only way I can open my projects now is by launching the Unreal Editor using its own executable icon, not through the launcher. That method always works. However, if I try to open the project directly from my computer folder or from the Launcher, I get the same warning box.
Honestly, this was a pretty expected bug from my perspective.
Same problem here, is a fix being worked on or what?
Thanks! I managed to generate an sln file and open the project. How did you determine that the error in UnrealVersionSelector.exe?
Just found the path in UE Sources, make small test game project with 1 function: “GenerateProjectFiles” like it’s done in Version Selector, attached debugger and see what’s going wrong. Before I worked with Version Selector (when I need custom engine build) and I already know: Launcher use VersionSelector (shared), custom builds use it, but there no Version Selector in Launcher UE builds.
only works if open the launch manually, and choose your project!
I get that, but Epic needs to release a fix for this, or is f-u UE4 users now?
Some of us have games that are coming out on UE4 and don’t care about UE5. What’s next? They turn off your ability to compile UE4 games? 
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Same here, after Updating game launcher, I can not open any projects on UE 4.27.2
This must be fixed. 
We have many projects on version 4 and are not planning to switch to version 5 until we finish the old projects.
For now, the only way I can open my projects is by launching the Unreal Editor using its own executable icon, not through the launcher.
This worked for me
Delete .vs, Binaries, Intermediate folders and .sln file
Copy the UnrealVersionSelector.exe file from “Launcher\Engine\Binaries\Win64” to “UE_4.27\Engine\Binaries\Win64”
Right-click → Run as admin → Register
Edit “UE_4.27\Engine\Binaries\DotNET\UnrealBuildTool.exe”
add this to the end of the file:
[PlatformPaths]
UnrealBuildTool=Engine/Binaries/DotNET/UnrealBuildTool.exe
Right-click .uproject file → “Generate Visual Studio project files” → Rebuild Solution
Right-click .uproject file → Open with → “UE_4.27\Engine\Binaries\Win64\UE4Editor.exe”
Now double-clicking the .uproject file should work
Does Epic even know about this? It’s been 9 days…
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I send bug report, but I can’t find the issue in Bug Tracker.
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Simply deleted Launcher version 19 and installed version 18. Everything started working. I generated an sln file and the project launched!
I am now receiving a new message when I start the Epic Launcher. It prompts me to repair the project’s path. I select “Yes,” but it fails to fix the paths every time.
Hi guy, add this to the end of the file:
[PlatformPaths]
UnrealBuildTool=Engine/Binaries/DotNET/UnrealBuildTool.exe
==
Please tell me the filename and file path , I use UE4 + VS
@DacTan88, see this (Config\***Engine.ini):
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Thank Alex, but I got a problem “Missing UnrealBuildTool.exe after build“
I use UE4 from Laucher and can’t Generate *.sln by right-click from .uproject file
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Guys, it is an easy fix. Just right click and click open with UE4EDITOR. Problem solved.