PHP Fatal error: Call to undefined function get_home_path()

We started getting errors when someone submits a new business directory listing. Here is the full error message from server log:

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to undefined function get_home_path() in /var/www/vhosts/mydomain.com/httpdocs/wp-content/plugins/user-spam-remover/user-spam-remover.php:93\nStack trace:\n#0 /var/www/vhosts/mydomain.com/httpdocs/wp-content/plugins/user-spam-remover/user-spam-remover.php(113): UserSpamRemover->__construct()\n#1 /var/www/vhosts/mydomain.com/httpdocs/wp-content/plugins/user-spam-remover/user-spam-remover.php(924): UserSpamRemover::getInstance()\n#2 /var/www/vhosts/mydomain.com/httpdocs/wp-includes/user.php(3467): wp_new_user_notification()\n#3 /var/www/vhosts/mydomain.com/httpdocs/wp-includes/class-wp-hook.php(324): wp_send_new_user_notifications()\n#4 /var/www/vhosts/mydomain.com/httpdocs/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()\n#5 /var/www/vhosts/mydomain.com/httpdocs/wp-includes/plugin.php(517): WP_Hook->do_action()\n#6 /var/www/vhosts/mydomain.com/httpdocs/wp-includes/user.php(3447): do_actio…', referer: https://www.mydomain.com/directory/?wpbdp_view=submit_listing