Here is a modified version of your code, where I changed Time to t (in the assignment of S), and I plotted your data as large individual points:
s1 = (1/2)*(((Vmax)*(Vmax))/a);
s2 = (Vmax)*(t2)+((1/2)*(na)*((t2)*(t2)));
h(2) = plot(tm, Vmax,'.');
You can see that each plot was only a single point. So, when you try to plot a line "between the points", nothing shows up because there are no lines to draw.