Sometimes when you update Unity and/or the Visual Studio Editor packages in Package Manager, the Visual Studio Editor package will get into a bad state where it won't recognize scripts in most assemblies. This often includes the assembly that's built from scripts in the Plugins folder, such as the Dialogue System, Love/Hate, and Quest Machine.
To resolve the issue, upgrade or downgrade the Visual Studio Editor package, or Visual Studio Code Editor package, depending on which Visual Studio editor you're using. This will usually reset the package's state. If that doesn't fix it, try updating Visual Studio itself. If you have the ability, back up your system before updating Visual Studio. Microsoft does not make it easy to downgrade Visual Studio once you've upgraded it. So if you absolutely must go back to an earlier version (which is extremely rare), the best way is to restore a backup of your system.