pot.energy

You would want to use the more general conservation of energy equation

KE_i + PE_i + W = KE_f + PE_f .

This takes care of situations where either an external force is doing positive work (from, say, some applied force) or negative work (from, for example, friction or air drag).

The work-kinetic energy theorem has more limited applications, as does the corresponding work-potential energy theorem (W = -\DeltaPE), since each assumes the other form of energy isn't changing.

If work is conservative, that means (among other things) that the sum KE + PE (called the total mechanical energy) is conserved, that is, remains constant. That tells us that W = 0 in the equation I gave, so it vanishes from the equation.

Non-conservative work means the sum KE + PE does change during the process; positive work will increase this sum, while negative work will decrease it. Various processes may cause that change only in KE, or only in PE, or in both of them. That's why this equation is more generally applicable than either of the work-energy theorems.

Copy paste

You see, the main idea behind defining a potential energy for the conservative forces is that the work done by these forces from one point to the other, is independent of the path and depends only on the initial position and the final position. In fact, the decrease in potential energy of a conservative force field is actually equal to the work done by that (conservative) force.
Now, if the force is not conservative, then the work done by this force will depend on the path. So, if you go from point 1 to 2 via path A, the force does some work, while you go from point 1 to 2 via some other path B, the force does some different amount of work. Obviously, since the works are not equal, we cannot express them as the change in some single function, since the change is not unique. That is the reason why the potential energy is not defined for non-conservative forces.

@kaymant sir
ok...even then we can define different potential energy coressponding to non-conservative force for different paths

Of course, you can. But that will be useless. For one thing, there will be infinitely many paths from one point to any other, and if you have to define a potential energy for each path, it will be as good if you don't define. If for each path you require a different P.E. why not simply calculate the work for each path.