Office for Mac 2011 – Outlook won’t accept my key

I have Office for Mac 2011 installed on my MBP running 10.11.5

I routinely use Excel, Word and PowerPoint with no problems.

I am trying to help a friend who is having problems with Outlook, so I tried to use my copy of Outlook for testing purposes. I have never used it before. After double-clicking the Oulook application, a dialog box appears asking for my key, which I thought was unusual, since I had already installed Office.

I tried entering my key, but it was rejected. What can I do to resolve this problem?

Simon