it's mainly a warning and the plugin should still work correctly.
more information on this specific issue with the turnitin plugin is here:
https://github.com/turnitin/moodle-plagiarism_turnitin/issues/155
There's a pull request here that fixes the error and seems to work ok for us:
https://github.com/turnitin/moodle-plagiarism_turnitin/pull/186