Copy the following ini command "bDisableStructurePlacementCollision=true" and put it into your game.ini under the [/script/shootergame.shootergamemode] section. You find the game.ini on your server in the following folder: ShooterGame/Saved/Config/WindowsServer/
These option is bugged at the moment. If enabled everybody can build on other peoples claimed land. This needs to be fixed by Grapeshot. Please keep an eye on the patch notes.