How do I test for players in a specific area on Bedrock Edition?

You need to use /execute

/execute @a[x=6,y=3,z=10,dx=1,dy=0,dz=1] ~ ~ ~ effect @s levitation 1 1 true

Within the target selector @a[] you specify the starter coordinates with x=, y=, z=, and then the distance with dx=, dy=, dz=. the dx dy dz is the difference, so if x=10 and dx=5 you test for an area from 10 to 15, and this works the same for the other once as well both for positive numbers and negative.