Is law of conservation of energy violated here?

“When a body is moving with uniform speed on a rough surface under the application of a constant external force and an equal and opposite frictional force, the work done by both the forces should be equal. as a result, the net work done by both the forces should be zero. however, we find that heat energy is also evolved in this process.where does this extra energy come from?”

Answer:

The work done is against friction only and if there is no friction, a force is not required to keep a body in uniform motion. The work done against friction appears as heat.