how is frictional force a non conservative force
Forces that do not store energy are called non-conservative of dissipative forces.
Friction force is an example of a non-conservative force because it do not store energy rather than storing it.
Frictional force oppose the motion of the object i.e.
Thus the net work done by frictional force is negative as,
Negative sign indicates that the frictional force continually drains energy from the object as it moves over the surface. So it dissipates energy in the form of heat.