I was having the same problem and I tried to manually open the UE4Editor (not using the launcher) and I got an error that it now REQUIRES MacOS 10.14.6 or later. I updated to 10.14.6 and it launches now.
Exact error says:
You can’t use this version of the application “UE4Editor” with this version of macOS
You have macOS 10.13.6. The application requires
macOS 10.14.6 or later.
Apparently, the minimum system requirement is now MacOS 10.14.6. I ‘wish’ Unreal would’ve just documented that.