If you have the Excel file open at the same time that Tableau is reading from the Excel file, you may get that error when you try to save the Excel file. Newer versions of Excel are somewhat better at managing this, but it can happen. You should not have Excel and Tableau accessing the same file at the same time.
Tableau discontinued the Legacy connector several versions ago. It is no longer available.
Does Tableau have a live connection to this excel file? Is there no error if Tableau isn't opened and using this file?