If you see The file couldn't open in protected view error while opening Microsoft Excel, the fixes listed in this post will help you.

How do I remove metadata from Excel on Mac? First, you must open the file from which you want the metadata removed. From there, click on the Tools menu, then select Protected Document. After that ...

Microsoft Office 2010 includes a new "Protected View" that, by default, will open all Outlook attachments and documents downloaded from the Internet in "read-only" mode.