Dear TomB_973,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
The steps you mentioned regarding to Password Protect Excel File, is only available in Excel for Windows OS(Not Excel for Mac OS) . To protect password Excel for Mac OS, here is steps for your reference: Protect a worksheet in Mac
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad