Su command permission denied

If you write su with the good password on your terminal and you have always the error "permission denied".It's probably because you user have not the group wheel.
So the only thing you have to do is :

gpassw -a user wheel

Why ?

The wheel group is a group which limits the number of people who are able to su to root. This usually consists of a group named “wheel” and a set of users that are permitted to use the utility ’su’ in order to change to root.