In order to get this achievement you need to establish 8 raider Outposts, not supply settlements as they do not count.
To get the ability to set up Outposts you need to start the quest "Home Sweet Home" which teaches you how to. The quest also requires you to set up 3 raider outposts which leaves 5 more outposts to do on your own.
Keep in mind that if you moved Shank to an Outpost and complete the quest you are prompted to return to Nuka World for the next main quest, Don't, instead talk to Shank to continue setting up Outposts and get the last 5 (If you want you can continue the main quest and then finish the 5 remaining Outposts afterwords).
If you never moved Shank to an Outpost when you go back to Nuka World talk to him you will be thrown into a fight with the
when the nearby enemies are dead you can talk to Shank to find out whats happening and will continue the main quest, But you can still talk to him to get the last 5 Outposts without having to do the main quest right away.
Once you've set up the 8th Outpost the achievement should pop right away,