I can't turn on the switch control it's was gray out

Try a "forced" restart of your iPad, using these instructions:

• On an iPad with a Home button: Press and hold the top (power) button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top (power) button. Only when the Apple logo appears (it might take 20-40 seconds), release that button (ignore all other screens that might appear).

If the issue persists, contact Apple Support, using this link:--> Get Support

Regards

Giulio

Perhaps you have restrictions enabled in Settings>ScreenTime>Content&Privacy Restrictions>Allow Changes...

Set all of them to "Allow".

Regards

Giulio