I have the same problem. I made a new component rectangular cube and wanted to cut one edge to 20°.
I created a sketch triangle and told it to do an extruded cut. IT WILL NOT SELECT THE RECTANGULER CUBE AND I CAN FIND NO WAY TO SELECT THE CUBE! This is relatively simple to do in other CAD programs. How do I select the "Object to Cut"?
I would have attached my file, but I don't know how to drag from the Fusion 360 cloud folders to my browser. I prefer not to keep my files in the cloud, so this may be due to me having no other experience using cloud storage.
Excuse my rift, but I am finding this program quite difficult to learn. It is no where near as intuitive as IronCAD! I even think that it is more difficult to use than Solidworks. IronCAD is my preferred program, but I have many years experience working with both IronCAD and Solidworks.
I bought a Fusion360 subscription to be able to do CAM. It works great with STEP214 files from IronCAD.
I felt that I should at least learn how to make simple items in Fusion360. I have wasted an inordinate amount of time trying to learn how to use this program! I don't know if it is because other programs have patent protection around certain procedures, but I just don't find Fusion360 to be very intuitive. It seems that once I finally do learn how to design something in Fusion360, it takes me two or three as many actions than are required in IronCAD. I don't think that program familiarity is the main reason for this.