A solution often has multiple 'projects' within it. If you cloned the code from Github or elsewhere and try to run it you may get that error. You just need to set one of those projects to the startup project to get it to run. This will often be the one that has API or UI in the name of the project. In the solution explorer, right click that one and select 'Set as Startup Project'. The rest of the projects may only be class libraries which can't be run.