We've seen a bunch of reports like this in the community, and in all those cases the reason was a (pre-flashed) SD card that apparently got flashed with a development version of OctoPi 0.18 from before it was switched over to Python 3.
You have two options to solve this.
One is to make a backup, reflash to the stable release version and then restore the backup.
The other is to migrate the Python environment on the card from version 2 to 3.
Either way, you will have to take some action, the report you are getting about still being on Python 2 is not a mistake.