The week 6 secret battle star in Fortnite season X is located in Fatal Fields. Watch this small video for the exact location:
In this map you can see the exact location of the week 6 secret star:
Once you arrive in this location go to the end of the pier and collect the star:
Be sure to collect the loading screen for Week 6 first. You can do this by completing the week 6 LTM missions.