This is part of a larger issue with mobile. All Input Trigger controls show up as little button on the screen that are not movable and you cannot use the normal buttons to trigger any actions. The whole system needs redone on mobile.
See my suggestion on how to do it correctly here: The Input Trigger System on Mobile is a Terrible Experience and Needs a Rework
Or on reddit here: https://www.reddit.com/r/FortNiteMobile/s/Mw0AeWOBY1