This happens if you've installed Office on more computers than your license allows. You may need to deactivate an old device.
A product key is a 25-character alphanumeric code used to "activate" your software. It serves as proof of purchase and ensures that your copy of Microsoft Office is genuine. Without a valid key, the software will operate in "Reduced Functionality Mode," meaning you can view documents but cannot edit or save them. ms office 2016 product key