Example 1: you can make it compile
Example 2: you can't make it compile at all
This forest-code is used as an example to show, what you can prepare, when compilation continues producing errors.
Y1: as posted
The following code was created somehow and posted, which obviously doesn't pass the syntax check in 3.1. It doesn't compile and complains about "missing forest":
\begin{forest}
for tree=draw,
[Main(), [Level Setup],[Game], [Win_Game()]]
\end{forest}
Y2: following the checklist
Though the code starts looking more like a MWE, it still doesn't compile:
\documentclass[10pt,border=3mm,tikz]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
for tree=draw,
[Main(), [Level Setup],[Game], [Win_Game()]]
\end{forest}
\end{document}
At this point you may decide to post after completing steps 3.3 and 3.4 as good as possible, and ask for help.
As it turns out, the forest code still has some errors in itself. The difference to the code snippet is that you removed almost all guessworking about your code before you post.
To verify, you might e.g. replace the forest-environment by some code from the forest manual. If it compiles, you know the MWE status is fine. You can mention that together with posting the problematic code above.