How to fix this error? ScreenGui.Main:23: '}' expected (to close '{' at line 6) near '['

Player = game.Players.LocalPlayer
LS = Player:WaitForChild("leaderstats")
Stage = LS:WaitForChild("Level")
Txt = script.Parent:WaitForChild("Level")
StageNames = {
	[2] = "Stage 2 -  Don't get catched by the aliens!",
	[3] = "Stage 3 - Don't touch the toxic water.",
	[4] = "Stage 4 - Escape the vent!",
	[5] = "Stage 5 - BLADES!!",
	[6] = "Stage 6 - Don't Touch the drilling rigs!",
	[7] = "Stage 7 - Avoid the lasers!",
	[8] = "Stage 8 - Do not cross!",
	[9] = "Stage 9 - Jump on UFOs!",
	[10] = "Stage 10 - Being Eaten.",
	[11] = "Stage 11 - The stomach.",
	[12] = "Stage 12 - Jump on these things..!",
	[13] = "Stage 13 - Choose the right wall.",
	[14] = "Stage 14 - This is the only way out...",
	[15] = "Stage 15 - Avoid the sand!",
	[16] = "Stage 16 - Do not touch the cars!!",
	[17] = "Stage 17 - Almost there.."
    [18] = "Stage 18 - Do not touch the cactus!" 
	[19] = "Stage 19 - This rusty metal can kill you"
	[20] = "Stage 20 - Rainbow jumps."
	[21] = "Stage 21 - WE DID IT!"
}
function DoStage()
	for i = 1, string.len(StageNames[Stage.Value]) do
		Txt.Text = string.sub(StageNames[Stage.Value], 1, i)
		wait()
	end
	wait(4)
	for i = string.len(StageNames[Stage.Value]), 0, -1 do
		Txt.Text = string.sub(StageNames[Stage.Value], 1, i)
		wait()
	end
end
Stage.Changed:connect(function()
	if StageNames[Stage.Value] then
		script:WaitForChild("CheckPoint"):Play()
		DoStage()
	end
end)
DoStage()

DoStage()

You didn't finish putting in Commas, Try this as the StageNames

StageNames = {
    [2] = "Stage 2 -  Don't get catched by the aliens!",
    [3] = "Stage 3 - Don't touch the toxic water.",
    [4] = "Stage 4 - Escape the vent!",
    [5] = "Stage 5 - BLADES!!",
    [6] = "Stage 6 - Don't Touch the drilling rigs!",
    [7] = "Stage 7 - Avoid the lasers!",
    [8] = "Stage 8 - Do not cross!",
    [9] = "Stage 9 - Jump on UFOs!",
    [10] = "Stage 10 - Being Eaten.",
    [11] = "Stage 11 - The stomach.",
    [12] = "Stage 12 - Jump on these things..!",
    [13] = "Stage 13 - Choose the right wall.",
    [14] = "Stage 14 - This is the only way out...",
    [15] = "Stage 15 - Avoid the sand!",
    [16] = "Stage 16 - Do not touch the cars!!",
    [17] = "Stage 17 - Almost there..",
    [18] = "Stage 18 - Do not touch the cactus!" ,
    [19] = "Stage 19 - This rusty metal can kill you",
    [20] = "Stage 20 - Rainbow jumps.",
    [21] = "Stage 21 - WE DID IT!",
}

You missed some commas:

StageNames = {
    [2] = "Stage 2 -  Don't get catched by the aliens!",
    [3] = "Stage 3 - Don't touch the toxic water.",
    [4] = "Stage 4 - Escape the vent!",
    [5] = "Stage 5 - BLADES!!",
    [6] = "Stage 6 - Don't Touch the drilling rigs!",
    [7] = "Stage 7 - Avoid the lasers!",
    [8] = "Stage 8 - Do not cross!",
    [9] = "Stage 9 - Jump on UFOs!",
    [10] = "Stage 10 - Being Eaten.",
    [11] = "Stage 11 - The stomach.",
    [12] = "Stage 12 - Jump on these things..!",
    [13] = "Stage 13 - Choose the right wall.",
    [14] = "Stage 14 - This is the only way out...",
    [15] = "Stage 15 - Avoid the sand!",
    [16] = "Stage 16 - Do not touch the cars!!",
    [17] = "Stage 17 - Almost there..", -- here
    [18] = "Stage 18 - Do not touch the cactus!", -- here
    [19] = "Stage 19 - This rusty metal can kill you", --here
    [20] = "Stage 20 - Rainbow jumps.", --  here
    [21] = "Stage 21 - WE DID IT!"
}