For a start x does not vary in the loop, so you'd be storing the result of the loop always in the same element of cossum. Secondly, you must use integer indices as the error message says. There's no reason to use x as an index in any case. I assume that's what you wanted:
x=(7*pi)/4
k=12
for n = 1:k
cossum(n) = ((-1^(n))*(x.^(2*n)))/(factorial(2*n));
end
Note that you don't need a loop in the first place. You can use vectorised operations:
x=(7*pi)/4;
k=12;
n = 1:k;
cossum = ((-1.^n)*(x.^(2*n))) ./ factorial(2*n)
