This is a very basic functionality. But some one can spend hours searching in the web for a solution.
The issue here is when we add/edit a server script and we compile the object, (Applet/Business Component etc.) the server script won't get compiled. Also when we try to exit Siebel Tools, we get a warning similar to below;
The following scripts have been saved but not compiled into the SRF. Press "OK" to exit. Press "CANCEL" to stop exiting Siebel Tools
This can happen specially in Opportunity Business component, BusComp_PreSetFieldValue method, as there is a vanilla script added and when we try to modify code.
The cause of the issue is that, the inactive flag is on for this script.
To fix the issue, You have to go to Business Component --> Select the required business component ('Opprtunity' in above case). Then go to BusComp Server Script (You have to make this visible from View --> Options --> Object Explorer) and remove the inactive flag
Compile the object and it will work.