Project wont build after adding a C++ file to it

I tried to add a c++ file to my project to convert it to a c++ project because of a tutorial on multiplayer networking (- YouTube) This one, but now my project wont build
this is the error I get

Rebuild started…
1>------ Rebuild All started: Project: Landscape, Configuration: Development_Editor x64 ------
1>Cleaning LandscapeEditor and UnrealHeaderTool binaries…
1>Creating makefile for LandscapeEditor (no existing makefile)
1>UnrealBuildTool : error : Module ‘Engine’ (Engine) should not reference module ‘Landscape’ (Project). Hierarchy is Project → Engine Programs → Engine Plugins → Engine.
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\VC\v160\Microsoft.MakeFile.Targets(51,5): error MSB3073: The command “Z:\UE_4.26\Engine\Build\BatchFiles\Rebuild.bat LandscapeEditor Win64 Development -Project=“C:\Users\goopc\Documents\Unreal Projects\THINGY\Landscape.uproject” -WaitMutex -FromMsBuild” exited with code -1.
1>Done building project “Landscape.vcxproj” – FAILED.
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

Severity Code Description Project File Line Suppression State
Error Module ‘Engine’ (Engine) should not reference module ‘Landscape’ (Project). Hierarchy is Project → Engine Programs → Engine Plugins → Engine. Landscape C:\Users\goopc\Documents\Unreal Projects\THINGY\Intermediate\ProjectFiles\UnrealBuildTool 1

Severity Code Description Project File Line Suppression State
Error MSB3073 The command “Z:\UE_4.26\Engine\Build\BatchFiles\Build.bat LandscapeEditor Win64 Development -Project=“C:\Users\goopc\Documents\Unreal Projects\THINGY\Landscape.uproject” -WaitMutex -FromMsBuild” exited with code 6. Landscape C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\VC\v160\Microsoft.MakeFile.Targets 45