A family of Microsoft word processing software products for creating web, email, and print documents.
It would help to see a screenshot of the actual notification... there are at lest 3 possible reasons.
The simplest is that Protection has been applied. The tool on the Review tab will be darkened. Just click it to turn the Protection off.
If the workbook was edited by an Excel for Windows it's possible that a feature was used which isn't available in Excel for Mac, especially if the document is stored on a SharePoint server. In that case you'll need to contact the user to have them provide a copy which has the restriction removed.
It's also possible that a Password to Modify has been applied. In that case there would be a prompt when opening the file, though.