i have same problem,
site work correctly, in Manage it, MySQL, when i want to create database work done. but i want to create user for that , dont work,
see root mysql, in mysql.user that i send data to create is exist, but in privilages not exist.

and grant all permissions not work, all priv field of that user record are set to N

are you use php-fpm?

insert your config steps here.

i recommend to you, use debian 8, because there many help for this os and is easier to use.

but you can check it: https://www.centos.org/forums/viewtopic.php?t=5243