For security reason, FreeBSD only allows su to root user,
if user is member of wheel group. Wheel group is a special group for administration purpose. Add your normal user to this group using pw command using following:
# pw user mod username -G wheel

So to add user amir to group wheel run command as follows:
# pw user mod amir -G wheel
# groups amir

amir wheel

Now su will work for amir.

You can disable this behavior complete for all users (not recommended until and unless you trust ALL of users):
1) Open pam configuration file for su using text editor:
# vi /etc/pam.d/su

2) Look for following line and comment it out:
Line:
auth requisite pam_wheel.so no_warn auth_as_self noroot_ok exempt_if_empty
Replace with:
#auth requisite pam_wheel.so no_warn auth_as_self noroot_ok exempt_if_empty

3) Now all users can use su command.