The most common cause of not being able to enter grades in the gradebook, when it looks like you should, is beacuse you might have categories set up with no grade items in them. You can check if this is the issue by looking at the Gradebook setup page.
The categories have icons that look like folders. if there are no grade items underneat the category rows, then you need to add them.
The figure below shows a Gradebook setup page with no grade items.
The figure below shows a Gradebook setup page with grade items.
You can fix this by by adding grade items to gradebook categories.