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.