Like the debugging message states: add_to_log() has been deprecated. Use the following link as reference to update your logging api: https://docs.moodle.org/dev/Migrating_logging_calls_in_plugins
This commit (https://github.com/markn86/moodle-mod_certificate/commit/4cabcba435ac9ffa318d49db41fe3d4d27245079) seem to have replaced add_to_log with events. May be try updating your certificate plugin. And like always, do a through test before deploying on your production server.