I use Windows Mail and Outlook, so am Not an authority on Yahoo. However, perhaps factor-fiction is able to confirm if my advice is correct.
You need to ensure that 'Conversation' is dis-abled before 'sort by sender' can be activated.
Click the gear wheel on top right of open mail page and, in the drop down menu, click 'settings' > unclick 'conversation and then 'save' . Next click 'View' at right top of your Inbox E/mails and in the drop down menu click the option which you require.
Hans.