Why I don’t have center stage function on my iPad Pro 11 3rd generation ?

Make sure you have turned it on in Settings->FaceTime->Center Stage.

If the option does not appear there, open FaceTime once and start a call and immediately end it. Then go back to Settings->FaceTime and the option should appear.

For iPadOS15, Centre Stage settings are accessed from the Control Centre whilst a supported App is running. Swipe downwards from top-right of the screen:

Be aware that Centre Stage is only supported by 2021 models of iPad Pro, the iPad9 and iPad mini6.

You need to first have FaceTime active in a call, then from the upper right Control Centre pull/swipe down, tap the Video Effects CenterStage button that should be present.

Tap the link below

Use Center Stage on your iPad - Apple Support

Best of Luck to You!

I stand corrected. The option is no longer in Facetime Settings but in Control Center.

Thanks Michel and Lotus.