Never had that error in any of the setups with Graphite I have running.
No idea what it means or where it comes from, tbh.
I primarily used this guide to setup graphite, maybe try it out and see if that makes it work for you 
A distro agnostic guide to Graphite with venv (examples: RHEL8, Debian10, Ubuntu18) - Tutorials and Guides - Icinga Community