What does this c++ build error mean?

Using Rider and 5.1, just adding some blank c++ classes and then rebuilding by right clicking the project and choosing Build Selected Projects:

results in this error:

The specific line:
<Exec Command="$(NMakeBuildCommandLine)" condition="'$(NMakeUseOemCodePage)' == 'true' and '$(NMakeBuildCommandLine)'!=''"/>

Full build log:

Build started 1/28/2023 9:09:02 AM.
Logging verbosity is set to: Normal.Project "C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.sln" on node 1 (Games\R2NAlaska target(s)).
ValidateSolutionConfiguration:
  Building solution configuration "Development Editor|Win64".
ValidateProjects:
  The project "UE5" is not selected for building in solution configuration "Development Editor|Win64".
Project "C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.sln" (1) is building "C:\Users\javor\Desktop\R2NAlaska\Intermediate\ProjectFiles\R2NAlaska.vcxproj" (2) on node 1 (default targets).
Build:
  D:\UE_5.1\Engine\Build\BatchFiles\Build.bat R2NAlaskaEditor Win64 Development -Project="C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.uproject" -WaitMutex -FromMsBuild
  Running UnrealBuildTool: dotnet "..\..\Engine\Binaries\DotNET\UnrealBuildTool\UnrealBuildTool.dll" R2NAlaskaEditor Win64 Development -Project="C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.uproject" -WaitMutex -FromMsBuild
  Log file: C:\Users\javor\AppData\Local\UnrealBuildTool\Log.txt
  Unable to build while Live Coding is active. Exit the editor and game, or press Ctrl+Alt+F11 if iterating on code in the editor or game
C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Microsoft\VC\v170\Microsoft.MakeFile.Targets(44,5): error MSB3073: The command "D:\UE_5.1\Engine\Build\BatchFiles\Build.bat R2NAlaskaEditor Win64 Development -Project="C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.uproject" -WaitMutex -FromMsBuild" exited with code 6. [C:\Users\javor\Desktop\R2NAlaska\Intermediate\ProjectFiles\R2NAlaska.vcxproj]
Done Building Project "C:\Users\javor\Desktop\R2NAlaska\Intermediate\ProjectFiles\R2NAlaska.vcxproj" (default targets) -- FAILED.
Done Building Project "C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.sln" (Games\R2NAlaska target(s)) -- FAILED.

Build FAILED.

"C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.sln" (Games\R2NAlaska target) (1) ->
"C:\Users\javor\Desktop\R2NAlaska\Intermediate\ProjectFiles\R2NAlaska.vcxproj" (default target) (2) ->
(Build target) -> 
  C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Microsoft\VC\v170\Microsoft.MakeFile.Targets(44,5): error MSB3073: The command "D:\UE_5.1\Engine\Build\BatchFiles\Build.bat R2NAlaskaEditor Win64 Development -Project="C:\Users\javor\Desktop\R2NAlaska\R2NAlaska.uproject" -WaitMutex -FromMsBuild" exited with code 6. [C:\Users\javor\Desktop\R2NAlaska\Intermediate\ProjectFiles\R2NAlaska.vcxproj]

    0 Warning(s)
    1 Error(s)

I’ve gone through individual files and compiled each with no errors. Anybody got a clue?

Nuked the binaries, intermediate, and derived data cache directories and then rebuild.

In the editor or in rider, use ctrl alt f11 to build, rather than the default build. Because of live coding: