CSS incorrectly installed error

I bought a new server today, and when I added DarkRP to it, it says:

Counter Strike: Source is incorrectly installed!
You need it for DarkRP to work!

I don’t know how to fix this. Any ideas?

(User was banned for this post ("wrong section" - postal))

Install CSS.
Try solving your problems by yourself instead of asking us every single time you got a little problem.
If you’ve read the error you would’ve known what to do.