Error during event registration linked to GDPR extension
Its just the warning (Notice), either you can turn off php warnings for wordpress or update the GDPR Communications Preferences settings(http://domain.org/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm/gdpr/comms-prefs/settings&amp;reset=1) and choose something (eg Do Nothing) for 'Add to Event and Contribution Thank-you pages' option under 'Event &amp; Contribution thank you page' section.

HTH

Pradeep