"She's already bought the plane ticket."

Translation: 她已经买了机票。

[FifiTsao]

她已经买机票了。

[NasuSamaruk0]

Yes, that is also acceptable.

[Keith_APP]

As I pointed out once, the pattern Duo is using now V + 了 + Direct Obj. is better used as a clause rather than a stand alone sentence.

[Rk5I3]

But it was not when I tried it (as of Jan 20th 2018).

[MikaelTrob]

她已经买过机票了
Should also be correct, but it said I made a mistake

[Andrea22756989]

她已经买机票了 should also be accepted. Reported it the 11/02/2018