How do I make it if a player puts a certain text in a text box it makes a text label visible?

Your error is in line 24, like the error message says. You are doing 1 too many ".parent"s. ".Parent" Goes to the TextBox, and another ".Parent" goes to the frame.

New Code Line 24

script.Parent.TextLabel.Visible = true

All Code

local textBox = script.Parent

local secretWord = "Testing1"
local colorNormal = Color3.new(1, 1, 1) -- white
local colorWrong = Color3.new(1, 0, 0) -- red
local colorCorrect = Color3.new(0, 1, 0) -- green

textBox.ClearTextOnFocus = true
textBox.Text = ""
textBox.Font = Enum.Font.Code
textBox.PlaceholderText = "Write the text here"
textBox.BackgroundColor3 = colorNormal

local function onFocused()
    textBox.BackgroundColor3 = colorNormal
end

local function onFocusLost(enterPressed, inputObject)
    if enterPressed then
        local guess = textBox.Text
        if guess == secretWord then
            script.Parent.TextLabel.Visible = false
if script.Parent.Text == "Testing1" then
script.Parent.TextLabel.Visible = true
else
print("Test")
end
            textBox.Text = "TestRight"
            textBox.BackgroundColor3 = colorCorrect
        else
            textBox.Text = "TestWrong"
            textBox.BackgroundColor3 = colorWrong
        end
    else
        textBox.Text = ""
        textBox.BackgroundColor3 = colorNormal
    end
end

textBox.FocusLost:Connect(onFocusLost)
textBox.Focused:Connect(onFocused)