Whenever I try to edit or add a new newsletter I get

Whenever I try to edit or add a new newsletter I get the following notice...

Fatal error: Maximum execution time of 30 seconds exceeded in /home/allsaintsmedia/theleprechaunluau/wp-includes/cache.php on line 618

and this is the url

http://theleprechaunluau.com/wp-admin/customize.php?wp_customize=on&theme=Simplicitydark&newsletter_id=8&return=admin.php%3Fpage%3Dnewsletters