There are two main potential sources of the issue and it depends on the symptom. Is the propeller turning over, it just doesn't seem to ignite, or is the propeller not spinning when running the starter?
If the propeller is turning:
Verify the fuel selector is to L or R, and that the tanks have fuel
Prime the engine for max. 2-3 seconds before starting (full throttle, full mixture, then prime for 2 seconds or so. Too long can flood the engine.
Pull back throttle, Set the fuel pump to boost
Set throttle to 1/4 inch, in the virtual model. If you have a hardware throttle, this is going to be much less than 1/4", as nearly all sim throttles have significantly less throw than the real SR throttles.
Engage the starter. If it doesn't turn over within a few seconds, slowly oscillate the throttle up and down a bit.
If the propeller is not turning:
In the SR20 -> Aircraft window, charge the batteries
Perform maintenance via the maintenance tab of the same window.
You can also via the same menu change the starter type. You may have more luck with one over the other.