It is hard to notice this skyshard on top of the guard tower in Sentinel. If you take a close look at the screenshot above you will see the Sentinel Wayshrine bellow the tower in the background. You can’t actually get to the skyshard from up here. You have to enter the door on the ground floor of the tower to reach the top.
Go inside the tower on the ground floor …
… and use the door to get to the top.
Sand Kingdom Moons - Super Mario Odyssey Power Moons