iPad mini

You need to properly prepare the iPad for its new owner before giving it away. However, you can still release the device from your AppleID account; you’ll find all the information that you’ll need here:

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

Did you follow the link that I provided?

This excerpt describes what you need to do:

If you no longer have your iPhone, iPad or iPod touch

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.

Erasing your information does not release the Activation Lock - or remove the device association from your own AppleID account. You have to follow the documented procedure to properly prepare the device for re-use by a new owner.