Cameroon person in Thailand want to apply to visit China

I am from Cameroon, i work and live here in Thailand for 2 years now, i have my working permit and my non-b visa valid up to next year, when i went for a tourist L, to travel to china for vacation, they turn down my application on grounds that i need a letter from the local government where as i had one invitation letter from a company in china,inviting me to spent the 2 weeks vacation with them. so what can i do in order to have a letter of invitation from the government?