These errors are due to a bug in the Google Meet for Moodle plugin. I've reported it as #36 Undefined constant "MESSAGE_DEFAULT_ENABLED". You should be able to work around this error by editing mod/googlemeet/db/messages.php changing lines 30 and 31 to:
30 'popup' => MESSAGE_PERMITTED + MESSAGE_DEFAULT_LOGGEDIN + MESSAGE_DEFAULT_LOGGEDOFF,
31 'email' => MESSAGE_PERMITTED + MESSAGE_DEFAULT_LOGGEDIN + MESSAGE_DEFAULT_LOGGEDOFF,