When I selected Extruded Boss/Base command this message is coming. What is the reason for that?
Because you haven't preselected a sketch to extrude. It's asking you to pick a sketch to extrude or a plane to sketch one onto & then extrude.
It means you either have to select one of your existing planes to sketch the feature onto or select an existing sketch to use that you want to extrude. Have you tried doing the basic tutorials that are included with your software? They are very useful for basic elements such as this.
Maybe this might help 2016 SOLIDWORKS Help - SOLIDWORKS Resources
It seems you are new to SW, whenever you exit from the sketching mode and click anywhere in the drawing area and then you select the Extrude Boss/Base tool, it will prompt you to select the already drawn sketch or if you haven't drawn any sketch then you need to select any plane for a drawing a sketch. Hence this tool can be invoked before or after drawing a sketch. Also you can directly choose this tool without exiting from the drawing environment, that time it will not popup such message.
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