I tried to makes a new part in an assembly by making a new sketch. The plane of this sketch is in one of the existing part. However, after I finish of making the sketch, I only can cut extrude it and the other function button are unable to use. What I did wrong? I need to join extrude the sketch to make a new part.
Those features are for modifying the model on an assembly level - not for creating a new part. To create a new part from within assembly you have to select "Create Component" first and then proceed with the modeling.