Okay! I may have figured it out!! What version of Twine are you using? I just downloaded an older version of Twine and tried my template out and got the error, yay! (Yay as far as hopefully having figured it out haha, boo about it not working on certain versions of Twine lol)
It looks like it's only working for version 2.3.12 and up. I'm thinking that's because Sugarcube was updated to 2.34.1 in that version, so I'm not sure if it's due to Twine being outdated or Sugarcube technically. But I just downloaded the newest version of Twine (2.3.16) and it's back to working so I'm crossing my fingers that you're perhaps running a version older than 2.3.12??