Translation:Are you busy on weekdays?
Just to add info:
The び reading is basically just the ひ reading but with rendaku. It's a super common thing you'll see in a lot of words built from multiple kanji.
The に reading is specific to 日本 and I don't believe appears anywhere else. I assume it stems from the にち reading getting shortened.
And of course it's in some words like 明日 (あした) but you have to treat those as one unit, so you can't consider any part of that word as just a 日 reading.
If it helps, translating in order could be read as:
"The weekdays - busy are they?"
Thus doesn't sound as natural as it should in English when dropping the subject. However, if you said this sentence to someone they would infer that you were asking them from context just as you do when not including the subject in Japanese.
This is Duolingo's way of showing that you have moved ahead too fast. Some languages are just difficult and we need to move slowly. Instead of using the normal training, you can manually select a topic to practice. Pick one that is just about the right difficulty, it will count toward your streak just like the standard practice. Then work your way down the tree again at your normal speed, don't learn new lessons until you have caught up. Japanese is one of the world's hardest languages, so it is natural to have to slow down and repeat a lot.