MATLAB uses one-based indexing. Therefore, in your code, on the first iteration where i = 1, the line marked is trying to index x1 at the zero position:
for i = 1:12
if i == 12
x1 = 9
end
x1 = [1];
x2 = [3];
x1(i) = x1(i-1) - x2; % <-- error here
x = [x1 x2]
end
There are still issues with your code, but it's hard to know what the fix is without more context. Also, note that i is reserved as an imaginary number in MATLAB, so it's recommended to use ii or another variable as your iterator.
