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I suggest you recreate a Word VSTO Add-In project and build it to determine if the problem only occurs in a specific project.
If the issue only occurs in a specific project, you can try cleaning up the build of that project and rebuilding. You can try the following methods：
1. Close Visual Studio, open the project folder, delete .vs and all bin/obj directories, and then reopen Visual Studio to build. If you don't see the .vs folder under the project folder, you need to check the hidden items in the view settings to make it visible.
2. Clean up the nuget package. Turn to Main menu>Tools>Options>Nuget package management and click clear all nuget storage.
3. Open the project folder to find the assemblies referenced by the project and compare them to the referenced assemblies in Visual Studio to check for missing assemblies. You can also reinstall your assemblies in Visual Studio to avoid missing assemblies.
If the problem exists in other projects, then your Visual Studio may have an error. Try repairing Visual Studio using the Visual Studio installer.
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