Hi @Ieuan_ab_Arthur, I’m a user too
Recently, Memrise has released new versions of some courses (particularly courses 1 & 2 of various languages, and, in the case of the Japanese course, 0 & 1 & 2). You can read about it in the topic: New Year, New Courses!
At first there were language courses for British English speakers, but now they have already released for American English speakers as well.
After you complete the new courses (1 or 2), the course shows as incomplete, missing a few words (if you are only using web version). But the correct one is to show it is complete. Because of that, the “learn” button remains active, and if you press it, it may show: “Whoops! Could not load this session”.
This is happening because these new courses have grammar mode (which is in beta: see here), but grammar mode is currently only available in the app. Grammar mode contains some new words too, which can only be accessed through the app. So even learning all the words in the web version, the program will show that the course is incomplete.
The levels that can not be seen in the web version are the levels that have grammar mode in the app. While grammar mode is not created for web version, we need to deal with it. Then, it will probably be fixed over time. Just skip and continue your learning