Well, it varies from map to map, however, I commonly see this method for disabling saving... You bascially just scroll through the jass or use ctrl+F (find) and search for 'save' 'single' 'load' and check the conditions etc, otherwise, look for something that disables another trigger.
Example:
The two red lines are places where you would change something if you wanted to reverse whats happening, Firstly, you can change the false to true, so it always ends up true and you can save, secondly, you can delete the trigger line that disables the trigger.
Look for things like this and you should be able to enable single player in most rpgs, however, there are many methods of disabling saving etc.