Hi there, I’m a Salvadorian passport holder. can I apply for 72 hours visa?

Bladimir, Salvador isn't one of 51 countries whose citizens enjoy 72 hours free transit. However, your transit is less than 24 hours, so I think you still don't need to apply one because you meet the requirements for 24 hours free transit. You need to bring your passport, onward air ticket and entry permit for the third country or region before you land at Guangzhou Airport.