Cannot add site

When trying to add a site to the IWP client account I’m getting the following error:

PHP Fatal error occurred: Uncaught Error: Call to undefined function get_user_by() in /home2/wpclubs1/public_html/cciss/wp-includes/meta.php:1415 Stack trace: #0 /home2/wpclubs1/public_html/cciss/wp-includes/meta.php(173): get_object_subtype(‘user’, 1) #1 /home2/wpclubs1/public_html/cciss/wp-includes/user.php(856): update_metadata(‘user’, 1, ‘show_welcome_sc…’, 1, ”) #2 /home2/wpclubs1/public_html/cciss/wp-content/install.php(33): update_user_meta(1, ‘show_welcome_sc…’, 1) #3 /home2/wpclubs1/public_html/cciss/wp-admin/includes/upgrade.php(13): require(‘/home2/wpclubs1…’) #4 /home2/wpclubs1/public_html/cciss/wp-content/plugins/iwp-client/init.php(2188): require_once(‘/home2/wpclubs1…’) #5 /home2/wpclubs1/public_html/cciss/wp-content/plugins/iwp-client/init.php(2106): iwp_mmb_create_processed_iterator() #6 /home2/wpclubs1/public_html/cciss/wp-content/plugins/iwp-client/init.php(168): iwp_mmb_backup_db_changes() #7 /home2/wpclubs1/public_html/cciss/wp-content/plugins/iwp-client/init.php(3092): iwp_mmb_parse_request() #8 /home2/w in /home2/wpclubs1/public_html/cciss/wp-includes/meta.php on line 1415.

Can you help?

Thanks.