Mobile app lessons error

This looks like [Client App] WaniKani for Android, which has been discontinued.

Another option for Android is [Android] Flaming Durtles - Android app with offline support, which is still very much supported and constantly being improved upon by the creator. I use it, too, would recommend!

Otherwise you can find other options at the bottom of the list, here: The New And Improved List Of API and Third Party Apps