Translation:It's easier said than done.
I'm not sure if it's too late for this suggestion, but it would be cool if Duolingo had some lessons on some of the more common chengyu expressions. (p.s. I haven't finished the tree yet, so forgive me if the course actually does teach us those expressions, and I just haven't gotten to them yet)
Unfortunately it doesn't. Or fortunately, at least for now; given how alternative translations are handled, I'd think they've got more than enough on their plate now.
another common way to say this is: 说起来容易做起来难。shuo1 qi3 lai rong2 yi4 zuo4 qi3 lai nan2.