did you try a restart of postfix?
/etc/init.d/postfix restart
the maildrop folder should be created automatically upon restart of service.
or else you need to create the folder manually and restart the postfix service again.
mkdir /var/spool/postfix/maildrop
chown postfix:postdrop /var/spool/postfix/maildrop
chmod 730 /var/spool/postfix/maildrop