Don't do it on the system user.

app/models/user.rb

@@ -188,6 +188,7 @@ class User < ApplicationRecord
   end
 
   def send_welcome_tour_message
+    return if id == -1
     create_notification('👋 Welcome to ' + SiteSetting['SiteName'] + '! Take our tour to find out ' \
                         'how this site works.', '/tour')
   end