Help please.

Hi sir. I am Danish citizen and my family and i love to visit to china and mostly Lhasa area. I have family living in Lhasa and i have invitation from Them but visa Office didnt give us visa and asked US to make permit from travl permit. How and where Can i make that? Can we make it when we get to China. How long it takes? We have only 4 weeks all together.
Thank you for your time.
Pedersen