The "Discount" button is enabled at the main POS widjet:
To make a discount per order line: choose the line and click on the Discount button pointed above, enter the discount value (%), check the change in the order:
Check the discount at the bill:

To check the discount from the backend (available only for manager’s access):
PoS app > Orders > Orders > Choose the Order from the list > Open it > Check the discount: