If you get "su: Sorry for normal user account" under FreeBSD implies you are a normal user and cannot switch to root. To enable the current user to switch to root by 'su' command the user must be a member of the group "wheel".
You can add your user account to the wheel group by:
pw user mod username -G wheel
Example:
pw user mod Alice -G wheel
Btw you need to be root or have sufficient privileges to add a user to the group.