One thing that Rod and I notice with novice users of Excel is that they sometimes forget to hit enter when they are finished entering data in a cell or editing a cell.
When our students tell us that they can't do what we are asking them to do, the first thing that Rod and I suspect is that they haven't pressed the Enter key after they have finished entering data in a cell. When you are still in the act of editing a cell, there is a lot that you cannot do in Excel, because Excel thinks you are still in a data entry mode. Refer to the two images below and compare them.