You cannot fix it without changing what you are trying to do. You need to decide whether you are trying to define a formula for V(r), or if you are trying to assign V as an array of results. As a formula, you could have V(r) on the left hand side, but you would have had to not assign specific numeric values to r. If you are trying to assign V as an array of results, then you cannot use V(r) because you cannot index arrays at floating point numbers.
Your easiest fix would be
V=-K*(e^2./r)+ Alpha*exp(-r./p);
plot(r, V);
