Fatal Error wp_json_decode

Hi,

I am getting the following error when trying to access the add-ons page for the WPSchoolPress plugin:

Fatal error: Uncaught Error: Call to undefined function wp_json_decode() in /home/u406621260/domains/simple-school.com/public_html/wp-content/plugins/wpschoolpress/lib/wpsp-admin-addons.php:12 Stack trace: #0 /home/u406621260/domains/simple-school.com/public_html/wp-content/plugins/wpschoolpress/wpsp-class-admin.php(200): require_once() #1 /home/u406621260/domains/simple-school.com/public_html/wp-includes/class-wp-hook.php(310): Wpsp_Admin->wpsp_addons() #2 /home/u406621260/domains/simple-school.com/public_html/wp-includes/class-wp-hook.php(334): WP_Hook->apply_filters() #3 /home/u406621260/domains/simple-school.com/public_html/wp-includes/plugin.php(517): WP_Hook->do_action() #4 /home/u406621260/domains/simple-school.com/public_html/wp-admin/admin.php(259): do_action() #5 {main} thrown in /home/u406621260/domains/simple-school.com/public_html/wp-content/plugins/wpschoolpress/lib/wpsp-admin-addons.php on line 12

I have tried to fix this error myself, but I have been unsuccessful.

Could you please help me resolve this issue?

Thank you.