The German physicist named in the Kelvin–Planck statement is Max Planck.
The statement says that no device operating in a cycle can receive heat from a single thermal reservoir and convert all of it into a net amount of work. It is one formulation of the second law of thermodynamics. The name combines William Thomson, later Lord Kelvin, whose work helped formulate the principle, with Max Planck, who presented a closely related version.
This statement does not say that heat engines cannot produce work. They can, but a cyclic engine must reject some heat to a colder reservoir. That limitation distinguishes the second law from the first law, which concerns energy conservation. Rudolf Clausius supplied another famous formulation: heat cannot spontaneously flow from a colder body to a hotter one.
Max Planck made major contributions to thermodynamics before founding quantum theory. In 1900, his work on blackbody radiation introduced the quantum hypothesis, but the Kelvin–Planck statement belongs to classical thermodynamics and concerns the limits of heat-to-work conversion.