I’m having a similar problem on my German 1 course. I have 9 words to go, but I have already learned all of the grammar topics on the app. Regardless, every time I try to “learn new words” it brings up an error message.
It’s a problem with the grammar. The number of missing words is related to the number of ‘words’ connected to the grammar lessons. It’s maddening.
(And, in general, the lack of support and training on the grammar front is extremely disappointing. I like that the app is set up so that grammar is introduced subtly, in the background, and we learn usage before rules … but at some point the grammar actually needs to be taught. It’s by far the biggest problem with the Memrise ‘official’ courses. … The fact that there’s basically no grammar training at all after the first two levels is really quite ridiculous!)