I sold my ipad and new owner cant activate

Refer to this support page:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

“If you no longer have your iPhone or iPad

If the steps above weren't completed and you no longer have your device, use these steps:

1. Ask the new owner to erase all content and settings using the above steps.
2. If you're using iCloud and Find My [device] on the device, sign in to iCloud.com/find or the Find My app on another device, select the device, and click Erase. After the device has been erased, click Remove from Account.
3. If you can't follow either of the above steps, change your Apple ID password. This won't remove personal information that's stored on your old device, but it prevents the new owner from deleting your information from iCloud.
4. If you're switching to a non-Apple phone, deregister iMessage. 
5. If you're using Apple Pay, you can remove your credit or debit cards at iCloud.com. Choose Account Settings to see which devices are using Apple Pay, then click the device. Next to Apple Pay, click Remove.
6. Remove your old device from your list of trusted devices.
7. If you purchased an AppleCare plan for your device, cancel your AppleCare plan.”

Hi,

Please make sure First: "find my" off,

Secondly: log out from the iCloud of the iPhone you sold to the new owner,

Finally: Erase the iPhone.

After that, you give it to or sell it to the new owner.

or follow the step below:

What to do before you sell, give away, or trade-in your iPhone or iPad – Apple Support (AU)

LotusPilot wrote:

…

If you purchased an AppleCare plan for your device, cancel your AppleCare plan1. .”

PS: It might be possible to transfer an existing AppleCare plan: Transfer your AppleCare plan - Apple Support

Indeed - an observation that could be usefully passed to Apple, whose documentation the quote was directly extracted.