How to enable One handed mode in Xiaomi MIUI 11 when using full screen gestures?

You can not use the one-handed mode while using full-screen gestures.
As a replacement, you can use Quick Ball.

Go to Settings -> Search "Quick Ball" -> Quick Ball Settings -> Turn on Quick ball.

Now you should see a curved line around the left-hand side of your screen (last pic).
You can use the quick ball as a replacement for the one-handed mode. You may also drag is around the screen, or change its various settings to your liking.