I worked my way all through the Duolingo German course; sadly, I still speak German like a Patagonian... aye well.
In the last months, they introduced lives ('hearts') on the phone app which rendered it effectively useless; five mistakes and you're out for the day. Then they did the same to the web version

but I discovered by chance that if one registers as a teacher (or a student) the hearts go away. Which made it usable again...
(I know they can be removed by paying for the course(s). I don't object to that; I do object that a course that was advertised as being free changes the rules partway through...)
I also had a problem in that it wasn't in English. It was in American, which in spite of all the apologists is _not_ the same language. So much of the 'English' part of the course was predicated on knowing US terms, grammar, spelling, and general culture. Which on the whole I do, but translating DE-UK-USA-UK-DE was painful to me.
Neil