To add a password to an Excel document on Mac you need to select Save As

 

In the prompt that opens, select Options and enter the desired password in the field Password to open.

You’ll be asked to enter the password again

 

After clicking OK and Save the document will be saved with the selected password.

Make sure you remember the password, as it can’t be recovered.

In order to open the document, the password will need to be reentered.