How to Complete Beneath the Sands
The objective is to find the buried treasure. Summoning Scions is forbidden. In order to summon scions, you have to equip specific items. These items will list an Action ability, but with no job associated with that action. If you don't have any of these items, you don't need to worry about breaking this law.
Walk up to a sparkle and choose Investigate to find out if the treasure is buried there. The quest will end immediately as soon as you find the treasure, so if you want loot from the monsters, then deal with the monsters before you start checking the sparkles.
There are traps on the field, so use a Ranger's Awareness ability to reveal them if you don't want to trigger the traps.