Hi, I am very sorry to hear that! I want to make sure that the add-on works for everyone so here are a few possible fixes: Try to reload the tab or open YouTube in a new tab after installing. Check out the options, if the first option is disabled, you will still receive mix playlists when clicking on a video without a mix.

If those did not help, please open an issue on the GitHub:
https://github.com/Joooohannes/Less-Mixes-for-YouTube

or contact me by e-mail:
johannes.ji1702@gmail.com

I really hope you reply so I can solve your issue!