1. create empty child object to canvas
2. on the child add component toggle
3. now add button and see pop up message in full:

"Can't add 'Slider' to GameObject because a 'Scrollbar' is already added to the gameobject!
A game Object can only contain one 'Selectable' component"

4. now create a new child game object to the canvas and add component 'selectable' and then add component toggle. You'll now only see some of the message like this:

"Can't add 'Slider' to GameObject because a 'Scrollbar' is already added to the gameobject!"