Have 9th generation iPad, cannot turn off center stage in zoom. Any ideas?

Perhaps Zoom’s own support page will provide the information that you need:

https://support.zoom.us/hc/en-us/articles/360061159452-Using-Apple-s-Center-Stage-feature-with-Zoom

I had the same problem until today. When in zoom, turn the iPad into the portrait position and icons will come out on the left side. There is one to control center stage or portrait view.