why is my ( people mask ) not detecting the person ? Even on old pictures where I already have a people mask with edits.

I have the same issue. I am using Lightroom Classic 15.2 on a iMac 5k 2020. For a few weeks when i try to use mask with people, the loading circle spins for 2-3 seconds and then “no people detected”

When I go to the Masks options and click on people it is not finding the person for some odd reason. Even on old pictures where I already have a mask on of a person it is not allowing me to add a layer Mask of the person to edit something different on them. Why is that ? 

You need to give us more information. Your hardware, your OS, version numbers. A failure to detect people is (on Windows computers) often caused by outdated GPU drivers.