Stack doesn't actually allow indexing, and ElementAt is an extension method implemented by accessing Stack's iterator. My honest recommendation is to not use Stack, even if that's what the book was using. Stacks are not really meant to be traversed. They're meant to be pushed to, and popped (or peeked) from. Each ElementAt call requires O(N) time to retrieve. I recommend switching to List and implementing 'push', 'pop', and 'peek' as such:

// Instead of Push
scopes.Add(item);

// Instead of Peek
int peeked = scopes[^1];

// Instead of Pop
int popped = scopes[^1];
scopes.RemoveAt(scopes.Count - 1);

However, I understand that is a bit of a change. If you'd prefer to keep it as a stack, a simple reversal of the for loop can get the job done.

	// Resolve local variable assignment
	private void ResolveLocal(Expr expr, Token name) {
		// Decrement through scopes
		for (int i = 0; i < scopes.Count; i++) {
			// Pass amount of layers since declaration to interpreter
			if (scopes.ElementAt(i).ContainsKey(name.getLexeme())) {
				interpreter.Resolve(expr, i);
				return;
			}
		}
	}

That's enough to get these scripts running.

>for(var i = 0; i < 10; i = i + 1) { print i;} 
0
1
2
3
4
5
6
7
8
9

>var a = 0; var temp; for(var b= 1; a < 10000; b = temp + b) { print a; temp = a; a = b;}
0
1
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
6765

By the way, I noticed that redefining a global variable will crash as well, but a scoped variable will display an error as intended.