Why doesn't my script with IF and THEN work?

an if statement should be formatted like this

if foo == bar then
print('Hello World!')
end

EDIT:

so your code should be like this.

local speedRunWeather = game.ServerStorage.speedRunWeather
local t = game.StarterGui.changeGUI

if t.Text == "Hey, this is a text." then
speedRunWeather.Parent = game.Lightning
end

It also should be noted that when changing text on a gui, don't save the text as a variable,

Variables in RLua have many types (ex. Bool, String, Instance, etc.)

When you save a variable like you did before

Reference:

local t = game.StarterGui.changeGUI.Text

It saves the Text property of the Instance as a String variable. This means that when you change it later in your code, it isn't changing the text that is on the TextLabel, it is changing the Text variable.

Example:

local textLabel = Instance.new('TextLabel')
textLabel.Text = 'foo'

local Text = textLabel.Text -- The variable "Text" now equals 'foo'

print(Text) -- Prints 'foo'

textLabel.Text = 'bar' -- We set the TextLabel's text to 'bar'

print(Text) -- Still prints 'foo'

The way to do it correctly is to save the Text variable as the TextLabel's Instance. (Or just reference the TextLabel directly every time)

Example:

local textLabel = Gui.Notification
textLabel.Text = 'foo'

-- or just do

Gui.Notification.Text = 'foo'

Doing this allows you to change the instance's Text property directly, instead of what you did before (saving the text property as a variable).