If shaking your iPad does not show you the undo message please make sure that Shake to Undo is enabled on your iPad.
To enable Shake to Undo
Launch the Settings app on your iPad
Search for Shake to Undo
Open that settings section and turn it on
If shaking your iPad does not show you the undo message please make sure that Shake to Undo is enabled on your iPad.
Launch the Settings app on your iPad
Search for Shake to Undo
Open that settings section and turn it on