TextAdventure shows the Unity console error of "ArgumentException"

Here is a complete picture of the error.
Everything was working fine, until 9. Creating the use action, in the text adventure game part 2 tutorial.
I tried to play the game, but it wouldn’t load. I then went back to the code, and realized, that I missed a couple of lines of code, and also that I had accidentally written useDictionary.Add, instead of !useDictionary.Add. I went to fix it, but the error persisted.
I have no compiling errors in any of my scripts, and I only have that one error in the Unity console. Every time I hit play, another instance of the error appears in the console.

I really hope someone knows how to fix this problem, I have spent 3 hours on various types of solutions, but nothing has worked…

I have made a few different text adventures in C++ and C#. This is something we really need to see the code for, but according to your error, you tried to add a new entry to a dictionary using a key that is already in use. In a dictionary each key needs to be unique. Before you add to your dictionary you can check if the dictionary already is using that key, then you either do nothing or you alter the existing dictionary value at that key depending on what you want to accomplish.