Why does my output say "Workspace.Doors.Model.Main:5: attempt to index nil with 'Character'"?

You can not use localplayer on server-sided scripts, rather try making use of the Player parameter with it's set argument. So remove the game.Players.LocalPlayer and do

local Mag = (script.Parent.Center.Position - Player.Character.HumanoidRootPart.Position).Magnitude

Hopefully this helps

@DeUltimate23 it does not fix the issue, the output is not showing any errors for the door, but the door does not open. Here are the scripts used for the door:

LocalScript, located in StarterGui, ScreenGui:

local WaitLocalPlayer = game.Players.LocalPlayer.Character:WaitForChild("HumanoidRootPart")

local UIS = game:GetService("UserInputService")
UIS.InputBegan:Connect(function(i, g)
    if i.UserInputType == Enum.UserInputType.Keyboard then
        if i.KeyCode == Enum.KeyCode.E then
            for _, Door in pairs(workspace.Doors:GetChildren()) do
                local Mag = (Door.Center.Position-game.Players.LocalPlayer.Character.HumanoidRootPart.Position).magnitude
                if Mag <= Door.Range.Value then
                    Door.Event:FireServer()
                    break
                end
            end
        end
    end
end)

while true do
    script.Parent.ImageLabel.Visible = false
    for _, Door in pairs(workspace.Doors:GetChildren()) do
        local Mag = (Door.Center.Position-game.Players.LocalPlayer.Character.HumanoidRootPart.Position).magnitude
        if Mag <= Door.Range.Value then
            local D3ToD2 = workspace.CurrentCamera:WorldToScreenPoint(Door.Center.Position)
            script.Parent.ImageLabel.Visible = true
            script.Parent.ImageLabel.Position = UDim2.new(0,D3ToD2.X,0,D3ToD2.Y,0)
            break
        end
    end
    wait()
end

Normal script, named "Main", located in game.Workspace.Doors.Model

local Can = true
local Open = false

script.Parent.Event.OnServerEvent:Connect(function(Player)
    local Mag = (script.Parent.Center.Position - Player.Character.HumanoidRootPart.Position).Magnitude
    if Mag <= script.Parent.Range.Value then
        if Can then
            Can = false
            if Open == false then
                local Finish = script.Parent.PrimaryPart.CFrame*CFrame.Angles(0,math.rad(90),0)
                for i = 0,1,.1 do
                    local CFM = script.Parent.PrimaryPart.CFrame:lertp(Finish,i)
                    script.Parent:SetPrimaryPartCFrame(CFM)
                    wait()
                end
                Open = true
            else
                Open = false
                local Finish = script.Parent.PrimaryPart.CFrame*CFrame.Angles(0,-math.rad(90),0)
                for i = 0,1,.1 do
                    local CFM = script.Parent.PrimaryPart.CFrame:lertp(Finish,i)
                    script.Parent:SetPrimaryPartCFrame(CFM)
                    wait()
                end
            end
            Can = true
        end
    end
end)