Error BE0051: The specified project file does not exist: [projectFile]
This error occurs if a documentation source or a project reference refers to a project file that does not exist or a solution documentation source contains a reference to a project file that does not exist.
Check the documentation sources and references to ensure that the contain references to valid files that do exist. For solution documentation sources, you may need to load them in Visual Studio to determine whether or not they contain valid project references.