I'm guessing from the commands that you've used it's a v14.x appliance?! Otherwise you'd get an error when trying to install from jessie-backports (v15.x is based on Stretch; v14.x was based on Jessie).
You can check what TurnKey version like this:
turnkey-version
Assuming you are running v14.x, then your issue is different to this one. The v14.2 issue is documented here, you'll also find the workaround documented there in that thread too. You'll need to manually apply it.
Are you sure this is the same error? Are you running v14 or v15?
I'm guessing from the commands that you've used it's a v14.x appliance?! Otherwise you'd get an error when trying to install from jessie-backports (v15.x is based on Stretch; v14.x was based on Jessie).
You can check what TurnKey version like this:
Assuming you are running v14.x, then your issue is different to this one. The v14.2 issue is documented here, you'll also find the workaround documented there in that thread too. You'll need to manually apply it.
I hope that helps.