Step 1: Click on Checkout, then click on Behaviour and Edit Behaviour.
Step 2: Click on Behavior tab. Enable the Sticky Payment Button on Mobile button, if you would like to enable the sticky payment button on the mobile version of your checkout.
Step 3: Once you are done, don’t forget to click on Save changes
You can chose to display a sticky payment button on mobile version of the checkout by clicking on the Checkout tab in the menu of your Admin panel of the selected store.