If you can't insert new cells in an Excel spreadsheet and recieve an error message, make sure that you didn't copy any formating by accident.

If you cannot insert, add or create new cells in worksheets in Microsoft Excel, remove cell protection and unmerge the Rows to begin with.

I get an error dialog that says, “Cannot insert object.” (I can still add form controls, just not ActiveX.) I’m seeing reports of this problem with both Excel 2010 and Excel 2013.