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.
I've never heard of "packsack" before though :þ
I was happy that DL accepts "está en mi casa" = " is at home"
First world problems
"I have a comb in my rucksack, but my backpack is in my house."
Apparently this is correct, but using the word rucksack twice is wrong.
You should report this because it is clearly inconsistent to accept "rucksack" as a translation of "mochila" only once in the same sentence.
however could be used instead of but