The subplot indexing obeys the same rules as matrix indexing, that is the indices must be integers greater than 0. You are also not addressing the subplot calls correctly.
Try this:
n=1:10;
x=[1 1 1 1 1 1 1 1 1 1];
y=flip(x);
xe=0.5*(x+y);
for k = 1:numel(n)
subplot(2,5,n(k))
plot(xe)
end
Experiment to get the result you want, and please read the subplot documentation.
