Were the new French units even beta-tested? it's extremely frustrating to have obvious and natural equivalents ("au travail" here) not accepted because duolingo is just reversing the french-english translation
If you're really frustrated, maybe you could work on a different part of the course for a while? Already more correct variations have been added, thanks to cooperative work between users and Duo staff. So hang in there!
I think it's because of the different meaning of the verb. In this case "passé" doesn't mean "stopped by" or "happened", it means"spent" which is not a reflexive verb or one of the verbs that express motion like "arriver" or "rester", therfore "avoir" is used instead of "etre".