Invitation letter of duly authorized unit for F application

I was applying for business (F) in Chinese embassy Jakarta recently (the capital city), but I was
be rejected because I miss one letter call invitation letter of duly authorization unit. may I know how to apply this Invitation letter of duly authorization unit? that have any agency can help me with?
and my another question is, if I apply business F in China, that I also need Invitation of duly? thanks