The bolt holes are probably covered with undercoating and/or rust. Buy at least two wire brushes for cleaning the inside of copper pipe (1/2" ID) from your local big box home improvement store. It might help to cut the end off so you can chuck it into a drill.

 

Those are the right threads. This is not an uncommon problem, and i went thru the same thing when I tried to add a hitch to my them 4 year old Honda Odyssey. The treads are unplated steel, and rust quite quickly. You will need to purchase a metric 12 x 1.25 pitch tap and handle, and some tread cutting oil and slowly/carefully chase the threads. 1/4 turn in, 1/8 turn out to clear the cutters. After around 1/2 a revolution, pull it completely out and wire brush it clean. Repeat until you get two full rotations or so, and stop. The tap is tapered, so it is cleaning more than just the first two thread tracks. But the trick here is not to remove any more metal than you have to, or the bolts will fit too loose and risk ripping out.

My '08 Sienna was 3 years old when I bought it, and I added the hitch immediately. I still had to dig that tap out of my draw and do it to this van also.

 

Thank you for advice guys....i finally got it installed...took me 4+ hours with whole can of WD40 (special can with rust remover from pepboys) and wire brush...