I reckon you’re talking not about WaniKani itself, but this user script? That’s a problem the author of that user script would need to fix, not the WaniKani staff. But the script doesn’t seem to be supported anymore.
I took a short look into it, and it looks like the problem is with the table rows (tr) attributes. There are ids set while there should be CSS classes. As a hotfix (and clearly nothing more, because it’s not my code and didn’t do a thorough review), you can change that yourself: Go to line 238 in the code. There should be something like tr id=. Change that to tr class=, save it and reload your WaniKani tab.