I didn’t notice this before, but you need to change this to the path where Graphviz is installed. Is this part of R code that you have written yourself? If not, you need to find a way to configure R to use the correct path.
I might be able so give more detailed instructions if you can show your R code. Preferably, the whole program and data so I can execute it. If that is not possible; as large a portion of it has possible.