HumanoidRootPart is not a valid member of Camera?

This loop

for i,v in pairs(workspace:GetChildren()) do
end

gets all children of workspace, and the first child/object is Camera, because that is the first object under Workspace (first in ABC). And this:

local v1 = v.HumanoidRootPart.CFrame.p

I will write Camera instead of v, because that's the object it loops. This tries to set v1 to Camera.HumanoidRootPart, which doesn't exist, and that's the problem. If you are trying to get players' HumanoidRootPart, you need to detect if the loop object is a Model, if the model's name is a player's name, and then you can get it's HumanoidRootPart. If you want, I will edit this and write there how can you do that. Oh and I noticed something:

    if player.Name == player.Name then
        ...
    end

This is just unnecessary, because player.Name is player.Name, so.. Like if you are writing if "Cheese" is "Cheese":D Hope this helps! EDIT: There is your code that should work (tested and it worked, no errors and the TextLabel just spawned in my head :'D):

local billboardgui = game:GetService("ServerStorage"):WaitForChild("BillboardGui")
game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(char)
        local clonedgui = billboardgui:Clone()
        clonedgui.Parent = game.Workspace:WaitForChild(player.Name).Head
        clonedgui.TextLabel.Text = player.Name
        clonedgui.TextLabel.TextColor3 = Color3.fromRGB(255,255,255)
        for i,v in pairs(workspace:GetChildren())do
            if game.Players:FindFirstChild(v.Name) then -- if there is a player called the model's/part's/ANYTHING'S name (you will need this):
                local v1 = v.HumanoidRootPart.CFrame.p
                local v2 = char.HumanoidRootPart.CFrame.p
                local max = 20
                if (v1-v1).magnitude <=max then
                    clonedgui.Enabled = true
                else
                    clonedgui.Enabled = false
                end
            end

        end
    end)
end)