"It is easy to forget."
Translation:Estas facile forgesi.
Finally, I have found the motto to place on my family arms. Facilas forgesi!
That doesn't surprise me - course maintainers couldn't add all possible permutations of every sentence that Esperanto allows.
Generally, on Duolingo, it's best not to turn adjectives into verbs too much or to use less common word orders.
You can report it if you wish but I'd recommend modifying your language instead :)