Why can't machines convert all of the work put into the work coming of it?

When work is done on a system, it is converted into energy. There is no such thing as a machine that is 100% efficient. In a machine, work is converted into energy. Energy comes in many forms, including sound, heat, and light energy. In addition to these types of energies, there is an internal property of a system called internal energy which is always present. For a machine to function, there is typically some form of friction. When friction is present, some energy is lost from the machine as heat. Therefore, some of the energy is converted into internal energy. If all of the energy was converted to work, that would mean there is no internal energy in the system. This is impossible, meaning all energy cannot be converted into work.