I am using "Integration Options" as a reference to implement a custom command to switch from 2D action game to Naninovel.
So, when I try to perform command operations on assets used in 2D action, I get the following error.

error CS0246: The type or namespace name 'MoreMountains' could not be found (are you missing a using directive or an assembly reference?)

When I checked this, I found that the reference was not registered, so I added the reference in Visual Studio.
Here is an image of the addition (sorry for the Japanese).

However, the error disappeared in Visual Studio, but it remained in Unity.
How can I make the reference addition work in Unity as well?

I feel like this is more of a Unity question than a Naninovel question.
I'm sorry if this is the wrong question.

I hope you can help me.

Unity uses its own dependency management system for assemblies: https://docs.unity3d.com/Manual/ScriptC ... Files.html

But that system is only used when you create an .asmdef file and put your scripts in the same directory or under it. You've probably placed a custom script or modified an existing one inside Naninovel folder, which is not recommended.