Translation:Where can we go to buy hats?
I had what you had verbatim "where can we go to buy hats". Agree, should be accepted
As the plural form of objects is not used often in Chinese, "Where can we go to buy a hat?" should also be accepted. Here, 帽子 could refer to a hat, many hats, or just "hat" in general.
March 18, 2018: "Where can we go to buy hats?" is still not accepted. Crazy.
What's really crazy is that you're a time traveler. But on my today of March 15 2018, "Where can we go to buy hats?" is not accepted.