Wordpress developer fix bug

Hi, I want to know where $action is defined and need to fix bug. It's not working after ugrade php version to php 7.4. This action should be work when user logged out too. if ( is_user_logged_in() ) { // If no action is registered, return a Bad Request response. if ( ! has_action( "wp_ajax_{$action}" ) ) { wp_die( '0', 400 ); } /** * Fires authenticated Ajax actions for logged-in users. * * The dynamic portion of the hook name, `$action`, refers * to the name of the Ajax action callback being fired. * * @since 2.1.0 */ do_action( "wp_ajax_{$action}" ); } else { // If no action is registered, return a Bad Request response. if ( ! has_action( "wp_ajax_nopriv_{$action}" ) ) { wp_die( '0', 400 ); } /** * Fires non-authenticated Ajax actions for logged-out users. * * The dynamic portion of the hook name, `$action`, refers * to the name of the Ajax action callback being fired. * * @since 2.8.0 */ do_action( "wp_ajax_nopriv_{$action}" ); } Thank you