Log in to your back-office
Click on the cogwheel icon
Click on the POINTS OF SALE tab.
Click on the + next to the relevant point of sale type to expand it.
Scroll down to the Sale Type section, then click on the Dine-in tab.
In the Payment Methods sub-section, disable the toggle to the left of Pay on-site.
Click on Validate.
The "Pay later" button will then disappear from the ordering process: the customer is required to pay online at the time of the order.