for n=0:16 is the correct syntax, you are also making another error by doing the summation in every iteration, you should instead find all values and do the summation after the loop ends
x = 1.8
xl=zeros(17,1); %preallocation of the storage
for n=0:16
xl(n+1) = ((x^n)/factorial(n)); %save every iteration result
end
xs=sum(xl) %calculate the sum of the iteration results
it can also be done without loops
n=0:16;
xs = sum(((x.^n)./factorial(n))); %notice the dots
