This happens only on 4.21 via Launcher.
UE4 editor running with "DebugGame Editor" configuration fails to launch
The error log is below.
I guess the "Debug" path is incorrect and "DebugGame" is correct.
Can't find: \UE4\ Debug \Launch\Launch.precompiled
Can find: \UE4\ DebugGames \Launch\Launch.precompiled
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-66753 in the post.