Problem to activate on-access scanning

Hello,

I can not activate the on-access scanning.

Here is how I installed sophos:

Do you accept the licence? Yes(Y)/No(N)
> yes
Where do you want to install Sophos Anti-Virus? [/opt/sophos-av]
>
Warning: There is another installation of Sophos Anti-Virus on this computer.
Do you want to enable on-access scanning? Yes(Y)/No(N)
>
Sophos recommends that you configure Sophos Anti-Virus to auto-update.
It can update either from Sophos directly (requiring username/password details) or from your own server (directory or website (possibly requiring username/password)).
Which type of auto-updating do you want? From Sophos(s)/From own server(o)/None(n) [s]
>
Updating directly from Sophos.
Do you wish to install the Free (f) or Supported (s) version of SAV for Linux? [s]
> f
The Free version of Sophos Anti-Virus for Linux comes with no support.
Forums are available for our free tools at http://openforum.sophos.com/
Do you need a proxy to access Sophos updates? Yes(Y)/No(N)
> no
Fetching free update credentials.
Installing Sophos Anti-Virus....
Selecting appropriate kernel support...
When Sophos Anti-Virus starts, it updates itself to try to find a Sophos kernel interface module update.
This might cause a significant delay.
Sophos Anti-Virus starts after installation.
Installation completed.




Despite this installation, on-scanning is not enabled.