Are you sure janiel-core is installed correctly?
Please make sure you are using the latest version of janiel-core, as outdated versions can cause issues with fuel, keys, and item metadata.
This script has been tested on 3 different frameworks and with 3 different fuel, key, and inventory systems, and everything worked as intended during our tests.
If the issue still persists after updating and verifying the installation, please feel free to contact us on Discord so we can take a closer look and help you directly.