In material design when I try to access Toggle Button via Keyboard, the spacebar is used as an action button. Why not Left/Right keyboard keys are not used as action buttons.
What is the right key for Toggle Button if I access via keyboard?
Since toggle buttons are essentially just differently-styled checkboxes, it makes sense to use the same access key, which is normally Spacebar. (Here's an example of another toolkit's toggle button, which also uses Spacebar: Oracle JET.)
Depending on your UI toolkit and the particular grouping of controls in your UI, the arrow keys may be used to move to other toggle buttons in the same group, so personally I'd avoid over-riding those.