Chinese working visa for foreigners

Hello
I am a Cuban citizen. I'm currently in Cambodia and I want to apply for a Chinese working visa. I have working permit and invitation letter. I went to the embassy of China in Phnom Penh and they say I have to go back to my country to apply for this visa. They didn't say why. Is it true? Why I can't apply here. Is there any why I can apply for this visa from here?