How can I password protect and Excel file in Office 365? The steps for the Excel app don't work.

Try these steps:

Click on File in the top-left corner to open the Backstage view.
Select Info from the left-hand menu (this should be selected by default).
Click on Protect Workbook in the center pane.
Choose Encrypt with Password from the dropdown menu.
Enter a password when prompted, then confirm it by typing it again.
Click OK to apply the password.