This error occurs if no documentation sources are defined in the the project and the Additional Content Only plug-in is not present in the project or is disabled.

Check the following conditions which can cause this error:

• You need to include at least one assembly or Visual Studio solution/project file in the help file project as a documentation source. Add the assembly, solution, or project to the Documentation Sources project node in the Project Explorer window.
• If you are trying to build a help file based solely on additional or conceptual content, add the Additional Content Only Plug-In to the project via the PlugInConfigurations project property.
• If the plug-in is included in the project, make sure that it is enabled by checking the box next to it in the plug-in configuration dialog box.

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