Try rebuilding manually error

I created a C++ file and now I cannot open the project because of this error

Go into your project directory (Documents > Unreal Projects > YourProjectName) and Open YourProjectName.sln and launch then you should be able to see what’s going wrong.

Seen that suggestion other places I don’t seem to have one

Did you solved problem? If not you can take a look at below

Then go into your project directory and right click YourProjectName.uproject and click Generate Visual Studio Project files.