Invitation Letter for a welfare project in China

I am looking forward to go to China for a welfare project hrld by AIESEC. My contact person in China has sent me an Invitation Letter but Sri Lanka visa office says they need the Invitation Letter of duly authorised unit. They do not accept the Invitation Letter which has been given to me by my contact person in AIESEC china. What is the importance of the Invitation Letter of duly authorised unit, and is it a must to obtain that specific letter?