Thanks for the response.

I uninstalled two Microsoft Office files that I found in my Apps. I then reinstalled my subscribed version.
When opening an existing Excel file I have saved, I still get the same warning error as before: 'To edit files in this free version of Excel, they must be saved to OneDrive'

When following the instructions to save to OneDrive, I enter in my hotmail email address and once again nothing happens. It just reverts back to Excel with the same Enable Editing warning message.

I'm flummoxed as to why this can be such a problem. What is wrong?!