Add a graph. Set the simulation time to 20ms. The LM2596 model in Proteus often needs a few cycles to start oscillating correctly.
Remember: simulation is not a replacement for real-world testing, but it is the safest way to learn. Download the LM2596 library today, build your first switching regulator, and watch your 12V input efficiently drop to a steady 5V at 2A without burning your fingers. Lm2596 Proteus Library Download