Hi May,
I just had the same thing happen to me were when I tried to open Microsoft Word, a screen came up asking if I wanted to purchase Microsoft Office 365. Like you I didn't want to do this and wanted to continue to use Microsoft Office 2011.
Long story short, if you go into Finder, select Applications, and scroll down you'll see a "Microsoft Word.app" listed, and also a "Microsoft Office 2011 folder/icon". The copy of the Word app you are seeing at this level is the one that is bringing up
the screen asking you to purchase Microsoft Windows 365. Now if you select the Microsoft Office 2011 folder/icon, you will see another app down at the next level with the same name "Microsoft Word. app". This is your original 2011 app. Note, this situation
will be true for all your 2011 apps such as Excel, Powerpoint, etc.
Now to fix your problem, go back up to the Application level and delete the "Microsoft Word. app" at this level (and the Excel app, Powerpoint app, etc, while you're at it.) To the delete the app, just select Settings and Move to Trash. Btw, don't forget
to empty your trash afterwards, it will same you a lot of storage space on your hard drive.
Happy Computing,
Charlie