Error PRJ0001: Invalid or missing SHFBSchemaVersion
This error is caused by a missing schema version property in the project.
The most likely cause of this error is that you have attempted to load a project file that is not valid for use with the current release of the help file builder. Only projects using the newer MSBuild format can be loaded with version 22.214.171.124 and later. Use the project converter to convert older projects to the new format.
Another cause is loading an MSBuild project file that was not created by the help file builder. Depending on what the project file is for, you may be able to convert it to the help file builder format too.