APPLE MAIL - How to sort mail folders on my mac alphabetically ?

It is a little tricky. You have to drag until a line appears between existing folders, or it will drop into another folder.

You may her better luck if you move the folder to the left while you drag it. Dragging to the right may be interpreted as you intend to place it in another folder.

Drag the folders into the positions you want relative to each other, while singing the alphabet song.

Try not to drop a folder into another folder while you do this.