Hello JSDRC,
I understand that you're receiving an error message when attempting to turn off Family Sharing, and are not being removed from the family group that you're a member of. Alternatively, you may consider having the family organizer remove you from the group as outlined below.
The family organizer can remove anyone over 13 from the family group at any time.
In iOS 8 or later, tap Settings > iCloud > Family, tap the name of the family member you want to remove, then tap Remove.
In OS X Yosemite, go to Apple menu > System Preferences > iCloud > Manage Family, click the name of the family member you want to remove, then click the Remove (-) button in the bottom-left corner.
Children under 13 can't be removed from a family. If you need to, you can transfer the child to another family or delete their Apple ID.
Leave Family Sharing - Apple Support
Sincerely