The first entry in any kind of Matlab vector/array/matrix... is indexed with 1 (and not 0, as for example in C++). That is why you get an error in the first iteration of your for loop.
Two options:
adjust the indieces for df
for n = 1:10
df(n) = (f(x+(h^(n-1)) - f(x)))/(h^n-1);
end
or, in Matlab you can get rid or the for loop and matalb calculates the resulting array in a single line, in many cases this is faster than usinng for loops
n = 1:10;
df = (f(x+(h^(n-1)) - f(x)))/(h^n-1);
