How do I make a gui inserted when a player walks over an object?

You have a lot of problems with your script but I'm just gonna fix the main ones for you.

workspace.Hill1.NamePlate.Touched:Connect(function(hit)
    local player = game:GetService('Players'):GetPlayerFromCharacter(hit.Parent)
    if player ~= nil and player.claim.Value == false then
        if game.Workspace.Hill1.Claimed.Value == false then
            local claim = Instance.new("TextButton", player.PlayerGui.ScreenGui)
            claim.Position = UDim2.new(0.408, 0,0, -100) 
        end
    end
end)

First off .Touched passes whatever object touched it as an argument, not the player, so I just fixed that to actually get the player. You also were doing parenting the TextButton to StarterGui instead of the actual PlayerGui.

Hope this fixed your problem!

Sorry Here is how the code block should look like

game.Workspace.Hill1.NamePlate.Touched:Connect(function(player)
    if player.claim == false then
        if game.Workspace.Hill1.Claimed.Value == false then
            local claim = Instance.new("TextButton", game.StarterGui.ScreenGui)
            claim.Position = UDim2.new(0.408, 0,0, -100) 

        end



    end

end)

that block of code was in a local script Here is the script to insert the boolean value into the player (which works) it is a normal script