Hi,
The type ‘Mesh’ is part of UnityEngine and shouldn’t be bound to any scripting backend. I myself also use it as a backend for my projects without any issue. Do you get any other errors? If it can’t find UnityEngine.Mesh, it should pop up more errors, since it is used in a lot of scripts.
If you create a new script outside the Modular3dtext folder, can you create a variable:
Mesh mesh = new Mesh();
in that script.
Maybe the il2cp backend got corrupted for some reason. Can you clear the cache please? ( instruction )