Reason : permission not allowed
Method:
Step 1: chown
-R www:www your-wordpress-dir because
default user group of nignx is www ( /usr/local/php/etc/php-fpm.conf point to default user and user group)
Step 2: (key) : remove scandir function indisable_functions of /usr/local/php/etc/php.ini
Step 3: restart server, if you use lnmp, like this ./lnmp restart