I have a spreadsheet in Excel 2013, and wish to create a chart.

One does this by selecting the desired chart type from the Charts menu, which is part of the INSERT menu ribbon.

But in my case the Chart menu is greyed out. It does not allow me to insert any graph at all.

The Illustrations menu (Shapes, etc.) is also greyed out.

What's going on? I have other spreadsheets of the same type (*.xlsx or *.xlsm) in which all menus on the INSERT ribbon are active.
I'm on a 2 year old Toshiba Tecra, with Win10 fully updated.