It's because of false inventory. Sometimes the website is not following the real partner inventory, and it errors out t the last step. You may call web support and let them find you a new routing or apply this routing to a new higher fare basis with available inventory.(they can waive your booking fee). I met this problem last month booking a UA-NH itinerary. Called web support and they gave me a new fare which is 1000 more than the original one.
I didn't take it, and gave my money to OZ.