That is an English error. You can't finish sentences with "off".
I use a "Two and a Half Men" episode to remember this.
Jake's teacher says something like "You don't flip people off, you flip off people"
Chinese-English / English-Chinese translation is not as precise as French to English, or English to Spanish. Add in Duolingo's inherent weirdness, and there will be issues. Tough it out as best as you can.