"The plane ticket to Italy is too expensive."
”去意大利的飞机票太贵了“ Still marked wrong as of 2019-03-15. Reported.
It seems like they actually have two separate exercises and in one you must use "飞机票“ and the other just ”机票." I would suggest to the Duolingo team that this be one exercise and accept either solution. The only reason I can think they would not do this is if they want to require that you answer audio questions in both forms.