is_ajax is deprecated since version 6.1.0 – WordPress

I got the same error and I fixed it by adding these lines in ‘wp-config.php’ file.

Insert BEFORE this line That’s all, stop editing! Happy blogging.

// Enable WP_DEBUG mode
define('WP_DEBUG', true);

// Enable Debug logging to the /wp-content/debug.log file
define('WP_DEBUG_LOG', true);

// Disable display of errors and warnings 
define('WP_DEBUG_DISPLAY', false);
error_reporting(0);
@ini_set('display_errors', 0);