It seems that there’s some conflict with one of the scripts. I had same situation just a moment before, and the item leveled up properly when I temporarily disabled scripts.
This is somewhat weird. I checked my scripts and the one that was updated the most recently was Niai Similar Kanji (7 days ago) and the rest was over a month ago so it seems it’s not the script change but something WK did?
From my troubleshooting it seems that “Wanikani Double-Check” is responsible (issue goes away after disabling this one)
I do not use Double-check, yet the problem persists. Disabling Tampermonkey or turning on compatibility mode seems to be the only solutions provided by others so far.
In fact, not only can I not do the last review without doing the above, but I get the message ‘Session has timed out’ when obviously I have been using the site continuously.