You try to use a command on the Ribbon, which is a part of the Microsoft Office Fluent user interface, but the command is disabled or unavailable.
The grace period for using your trial version of the 2007 Microsoft Office system, which is typically 30 days, has expired and the program is now in Reduced Functionality Mode. In Reduced Functionality Mode, you cannot create, save, or modify Microsoft Office documents. You can only view the documents.
If you have been using a trial version that is now in Reduced Functionality Mode, to regain full functionality you must convert your trial version to a fully licensed version of the product. You do this by purchasing a Product Key online or from a retailer, and then by using that Product Key to activate the product. For more information on product activation, see Activate Microsoft Office programs.