Sometimes, an Excel file will be in a read-only state if editing restrictions are in place or the file is not saved to a location that is considered safe.
If you have a "read-only" message bar across the top of your file or, for one reason or another, can't make any edits to your file, try completing a "File, Save As" and saving a copy of the file.
Alternatively, you might have issues editing an Excel sheet protected by the file author. Sometimes, parts of spreadsheets are protected by the author (typically your instructor) on purpose. If you believe the file protection is preventing you from completing your assignment, please contact your instructor directly.