Difference between NO2 and NOx

Fatal error: Uncaught Error: Call to undefined function learndash_lesson_hasassignments() in /home/customer/www/ecrd.in/public_html/wp-content/themes/buddyboss-theme/learndash/ld30/topic.php:259 Stack trace: #0 /home/customer/www/ecrd.in/public_html/wp-content/plugins/sfwd-lms/includes/class-ld-cpt-instance.php(705): include() #1 /home/customer/www/ecrd.in/public_html/wp-includes/class-wp-hook.php(307): SFWD_CPT_Instance->template_content('') #2 /home/customer/www/ecrd.in/public_html/wp-includes/plugin.php(191): WP_Hook->apply_filters('', Array) #3 /home/customer/www/ecrd.in/public_html/wp-includes/post-template.php(253): apply_filters('the_content', '') #4 /home/customer/www/ecrd.in/public_html/wp-content/themes/buddyboss-theme/template-parts/content-sfwd.php(15): the_content() #5 /home/customer/www/ecrd.in/public_html/wp-includes/template.php(772): require('/home/customer/...') #6 /home/customer/www/ecrd.in/public_html/wp-includes/template.php(716): load_template('/home/customer/...', false, Array) #7 /home/customer/www in /home/customer/www/ecrd.in/public_html/wp-content/themes/buddyboss-theme/learndash/ld30/topic.php on line 259