If, for some reason, $USER is not set, you can use the id command to obtain the identity of the real user.  So the first time you use the $USER variable, you can use the shell expansion to supply a default value.  Change the chown line in your script to:
sudo chown $"${USER:=$(/usr/bin/id -run)}:$USER$USER" "/var/www/$sitename$sitename"
 If USER is empty or unset when this is run, bash will set the USER variable to the output of /usr/bin/id -run.
 
                