enable SMART for HP hard disk

The most likely reason is that your server came with a hardware raid controller and sda is not an individual disk, but a logical drive.

HP provides tools to monitor your hardware, the typical Linux utility to monitor HP raid controller status and the drives contained would be hpacucli found here.

Some useful commands would be:

hpacucli> ctrl all show config 
hpacucli> ctrl all show config detail
hpacucli> ctrl all show status

hpacucli> ctrl slot=0 pd all show 
hpacucli> ctrl slot=0 pd 1 show
hpacucli> ctrl slot=0 pd all show status

To add to what Aleksandr said (I can't comment yet, unfortunately):

smartctl -x -A -d sat+cciss,0 /dev/sda

The sat+ bit gets you more than just cciss on its own - in particular, you should be able to access the individual attributes, not just the overall SMART status and computed values.

You can also use repos as write here - http://sysadmin.te.ua/linux/hpraid-monitoring.html

echo 'deb http://hwraid.le-vert.net/ubuntu precise main' > /etc/apt/sources.list.d/raid.list

This repo has almost all raid monitoring utils for all type of raids! Also article shows how to monitor HP raid using 'smartctl' as well.

smartctl -a -d cciss,0 /dev/sda

Regards.