This is what the result Missing["NotAvailable"] result indicates. It means we do not have the data for those inputs. I suppose you could programmatically check substance, particle, property combinations to find the gaps. There may be some difficulties with ensuring you remain within the valid energy range but StoppingPowerData will alert you via a StoppingPowerData::orng message in those cases.

Unfortunately StoppingPowerData does not currently include data for proton impacts on carbon. As you've notice data for that element is rather limited at the moment. We are always expanding our capabilities and we will work to add more data to cover this and other substances in the future.