They accept backpack but not pack. Check the Larousse spanish dictionary and they define: pack, rucksack, knapsack. They do not use the word bag. I think Duolingo should not confuse bag and pack.
I've never heard "pack" used as a noun on its own, only as a verb. Backpacks, rucksacks and knapsacks are frequently referred to as "bags" where I live. Backpack is perhaps the most common though.