Okay,
A couple of things.
First
You have two index pages. One is index.php the other is index.html
Index.php page : https://stressdefense.com/Training/Tai Chi/
index.html page: https://stressdefense.com/Training/Tai Chi/
I bet the link above for the PHP page has the changes.
So I’m guessing you added a stack to the page that requires PHP. You should have gotten a warning about having duplicate index files.
You need to find the directory on the server for that folder and then delete the index.html page using a file manager (if your host has one ) or an FTP app like FileZilla or Transmit.
Second
It’s really poor practice to use spaces in the folder name (or capital letters). Right now you have Training with a capital T and captial letters and spaces int the foler name /Tai Chi/.