First question is where are you looking for one or expecting it to be?

If you right click a message subject line it will be in that menu.

If you are looking for it on the Mail Toolbar and it is not there, right click the toolbar and select Customize. Find the Reply All button and drag it on to the Mail Toolbar.

If you are looking for it on the Message Pane or a Tab header you will not find one. What you will find is a Smart Reply button that shows as Reply when there is only one addressee on the message and a Reply All when there is more than one.

I put it on the toolbar, but it won't highlight. But I did figure it out by your third option. Thank you!!