Translation:The girl was not playing in the park with her friend last Tuesday.
Because it describes an action in progress (the girl was not playing), not told in the sense of a completed action (la fille n'a pas joué = the girl did not play). This is one of the key uses of the imperfect tense. Open this link in your browser to see an excellent explanation of this from the University of Minnesota: https://languagecenter.cla.umn.edu/lc/FrenchSite1022/FirstVERBS.html
Duo is playing with contractions now. In the past, this was automatically scored as correct but now it must be entered manually. The automatic contraction in all kinds of places was causing too many false positives so now they will be corrected one at a time. In the long run, it is better this way.