Not allowing my Realms to have add-ons be used

Hi, i have recently been trying to add different add-ons to be used on my realm shared with my friends. The add-ons i used were made by FoxyNoTail and his website is https://foxynotail.com/addons/ and the video i used to try to add them was https://www.youtube.com/watch?v=IH6Yf3W7Ceg&t=302s I made sure everything was right but nothing was working, I really would like a fix asap as we want this to be as close to a professional Java server as possible.