Hi Emcaseykay,
Thanks for contacting Apple Support Communities!
We understand that you're unable to activate Screen Time for your son's iPad.
Are you attempting to enable it via Family Sharing on your own device or on your son's iPad?
In the meantime, this is a handy resource which will show you how to set up Screen Time from your device or your son's iPad: Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support
If you're using Family Sharing to manage a child account
1. Go to Settings and tap Screen Time.
2. Scroll down and choose your child's name under Family.
3. Tap Turn on Screen Time, then tap Continue.
4. Set up Downtime, App Limits, and Content & Privacy with the limitations that you want for your child, or tap Not Now.
5. Tap Use Screen Time Passcode, then enter a passcode when prompted. Re-enter the passcode to confirm.
6. Enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
If you're not using Family Sharing to manage a child account
1. Make sure that you're on the device used by the child.
2. Go to Settings and tap Screen Time.
3. Tap Turn on Screen Time.
4. Tap This is My Child's [device].
5. Set up Downtime, App Limits, and Content & Privacy with the limitations that you want for your child, or tap Not Now.
6. Tap Use Screen Time Passcode, then enter a passcode when prompted. Re-enter the passcode to confirm.
7. Enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
We also want to be sure that the iPadOS is current as outlined here: Update your iPhone or iPad - Apple Support
We look forward to your reply.
Cheers!