WaniKani is currently transitioning the webpage to the React framework. Last week, they changed the Radical item pages, and it seems that today they have changed the Kanji pages. I am currently maintaining Keisei and Niai and will try to make them compatible to the new item pages as soon as possible.
EDIT: I have created a pull request with the fixes. It might take a few days until the pull request gets accepted. Afterwards, your script manager should update the script automatically.