For a system, the internal forces get cancelled. But does the work done also get cancelled?
For two bodies, the sum of the mutual forces exerted between them is zero from Newton's Third Law,
But the sum of the work done by the two forces need not always cancel, i.e. For example, consider the system as the block and the ground. If we consider a block being pulled on a rough surface, the work done by the frictional force on the block would be negative. But the work done by the frictional force on the ground would be zero. As we can see, their sum is not zero.