I’ve seen that in script canvas you can get your file extension wrong and it will check for what your file should have been named and load it successfully, perhaps Lua isn’t as forgiving. I see in the Lua code there is no .mtl extension used. You might see what you are using in SC for an extension and if it’s missing too, check whether you are getting a warning in the console when your graph runs.
Are you trying to set the material of the trigger area or the entity entering the trigger? In the graph you posted it will always set hexagon’s material. Here is a graph that is located on the same entity as the trigger area. The top changes the trigger area the bottom changes the entity that enters.
I haven’t tried setting materials in my own graphs but I do have a working example of changing material properties of an entity, where if you only want to change once instance of the material it is necessary to clone it first (otherwise every entity that shares the material will have share the change). Here I change material properties when a tag is added (and then lost to activate a toggle)