I can’t quit family sharing

If you have restrictions set on your account and the family organizer won't remove them, you can't leave without resetting your device. If you do the reset, you will lose any data not synced to iCloud. You will need to restore the device to factory settings. Do not restore from a backup made when the Screen Time code was active.

Restore to Factory setting.      

Leave your family group

Any family member over the age of 13* can remove themselves from a family group. *Age varies by country or region.

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > Family Sharing. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Family.
2. Tap [your name].
3. Tap Leave Family.

On your Mac

1. Choose Apple menu  > System Preferences, then click Family Sharing. 
2. Click Details next to your name.
3. Click Leave from Family Sharing.

If you're using macOS Mojave or earlier, choose Apple menu  > System Preferences > iCloud, then click Manage Family. Click [your name], then click the Remove (—) button.

When you leave Family Sharing, you lose access to any shared purchases or services shared by the family group.

Remove your account from a family group

If your Apple ID is being used to share purchases with a family group that you no longer want to be part of, you can visit your Apple ID account page and remove the account from the family directly. Go to appleid.apple.com and sign in with the Apple ID and password for the account you want to remove. In the Family Sharing section, choose Remove Account > Remove. After you leave the family group, you can start your own family group or join another one.

See these Apple documents on family sharing. Leave Family Sharing - Apple Support

You quit family sharing by leaving it. Leaving is covered in the heading and below it. Leave your family group.