The task is quite simple if you know where to go. There are a total of six different locations around the map to complete this mission and earn some experience points.
Here's how to complete the challenge "play the Spire's Message at a Guardian Outpost" in Fortnite.
How to Play the Spire's Message at a Guardian Outpost in Fortnite
To complete this objective, you have to first go to one of six locations around the map considered to be Guardian Outposts. These Outposts can be found to the north of Misty Meadows, north of Retail Row, southwest of Steamy Stacks, northeast of Pleasant Park, southwest of Pleasant Park, and in the Weeping Woods. These locations can be spotted and marked on the map, and you only have to go to one of them to complete the objective.
On arrival at one of the locations, head to the top of the tall outpost structure. There will be an object there for players to interact with, and once that's done, the quest will be completed. Be cautious of other players and enemies around the area, who can get in the way of you completing the objective.
Once done, players will be rewarded 15,000 XP.