Lighting ambience scripting issues (?)

while true do
end

This is an infinite loop that runs again right after finishing, it will run so fast that your code will just crash, your solution is to update ambience only when ClockTime property changes, in Roblox there is a function called GetPropertyChangedSignal, it returns you an event to which you can connect a function, this event will invoke your function every time the specified property changes. Your second issue in the code is that ambience and brightness values are numbers, in Lua numbers are copied every time you create a new variable, this means that

local a = 5
local b = a
b = 10
-- a: 5
-- b: 10
-- "a" and "b" are completely separate numbers

Doing ambience = 10 will only change the ambience variable and not game.Lighting.Ambient. To avoid this, you should not create a variable to these properties but just for the Lighting service. Also prefer using GetService over game.Service.

local Lighting = game:GetService("Lighting")

Lighting:GetPropertyChangedSignal("ClockTime"):Connect(function()
  if game.Lighting:GetMinutesAfterMidnight () > 17.8 * 60 then
    --game.LightingChanged.Ambient = Color3.fromRGB(154, 99, 15)
    print("it's 17:48")
    Lighting.Ambient = Color3.fromRGB(154, 99, 15)
    Lighting.Brightness = 4
  end
  if game.Lighting:GetMinutesAfterMidnight () > 19.3 * 60 then
    print("it's 19:18")
    Lighting.Ambient = Color3.fromRGB(112, 67, 0)
    Lighting.Brightness = 0
  end
  if game.Lighting:GetMinutesAfterMidnight () > 20.6 * 60 then
    print("it's 20:36")
    Lighting.Ambient = Color3.fromRGB(73, 44, 9)
    Lighting.Brightness = 0
  end
  if game.Lighting:GetMinutesAfterMidnight () > 5 * 60 then
    print("it's 05:00")
    Lighting.Brightness = 4
  end
  if game.Lighting:GetMinutesAfterMidnight () > 6.06 * 60 then
    print("it's 20:36")
    Lighting.Ambient = Color3.fromRGB(151, 110, 49)
    Lighting.Brightness = 4
  end
  if game.Lighting:GetMinutesAfterMidnight () > 6.4 * 60 then
    print("it's 20:36")
    Lighting.Ambient = Color3.fromRGB(179, 107, 0)
    Lighting.Brightness = 4
  end
  if game.Lighting:GetMinutesAfterMidnight () > 7 * 60 then
    print("it's 20:36")
    Lighting.Ambient = Color3.fromRGB(204, 132, 21)
    Lighting.Brightness = 4
  end
end)

Whenever you get better, try using tables to create a far shorter solution for this.

You made a lot of mistakes you should never do. Well first of all why it crashes (yes, it crashes) is bc you made an infinity loop without any delays. The easy solution is to put wait(1) somewhere in a script. If you game changes time of the day not smoothly (i.e. it doesn't use TweenService or something to cycle trought the entire day without skipping any second) I higly recommend you to listen for a Lighting:GetPropertyChangedSignal("ClockTime") for an instant response:

game.Lighting:GetPropertyChangedSignal("ClockTime"):Connect(function()
    -- Do your thing
end)

Second, you should use elseif/else in your script, otherwise it will look very weird when it applies changes to the ambience and then instantly changes it to something else. It prevents from multiple changes in the same time, do something like this:

if game.Lighting:GetMinutesAfterMidnight () > 20.6 * 60 then
    print("it's 20:36")
elseif game.Lighting:GetMinutesAfterMidnight () > 19.3 * 60 then
    print("it's 19:18")
elseif game.Lighting:GetMinutesAfterMidnight () > 17.8 * 60 then
    print("it's 17:48")
end

Important: check a time in a correct order (from highest to lowest) or else it will never go further after a first check like you did And the third mistake is that you are changin variables (on lines 9, 10, 14, 15...), not an ambience itself. To actually apply changes, do something like this:

local lighting = game.Lighting

if game.Lighting:GetMinutesAfterMidnight () > 20.6 * 60 then
    print("it's 20:36")
    lighting.Ambient = Color3.fromRGB(73, 44, 9)
    lighting.Brightness = 0
elseif game.Lighting:GetMinutesAfterMidnight () > 19.3 * 60 then
    print("it's 19:18")
    lighting.Ambient = Color3.fromRGB(112, 67, 0)
    lighting.Brightness = 0
elseif game.Lighting:GetMinutesAfterMidnight () > 17.8 * 60 then
    print("it's 17:48")
    lighting.Ambient = Color3.fromRGB(154, 99, 15)
    lighting.Brightness = 4
end

In the end you get:

local lighting = game.Lighting

game.Lighting:GetPropertyChangedSignal("ClockTime"):Connect(function()
    if game.Lighting:GetMinutesAfterMidnight () > 20.6 * 60 then
        print("it's 20:36")
        lighting.Ambient = Color3.fromRGB(73, 44, 9)
        lighting.Brightness = 0
    elseif game.Lighting:GetMinutesAfterMidnight () > 19.3 * 60 then
        print("it's 19:18")
        lighting.Ambient = Color3.fromRGB(112, 67, 0)
        lighting.Brightness = 0
    elseif game.Lighting:GetMinutesAfterMidnight () > 17.8 * 60 then
        print("it's 17:48")
        lighting.Ambient = Color3.fromRGB(154, 99, 15)
        lighting.Brightness = 4
    end
end)

Hope it's clear to understand. I actually had a few mistakes in the beggining of my scripting like yours, you just need to get used to it