how to make For loop for Invisibility power up?

In this kind of situation you need to loop through the body parts and check the part's Class the 2 Known classes that have the property Transparency are

1-MeshParts 2-Parts

We will have to use the for loop in maintaining the body parts and it can be implemented in the following code as i'll explain in big chunks of code;

Grabbing the user's Data/ Variables:

repeat wait() until game.Players:FindFirstChild("Divistern")
local player = game.Players.Divistern
repeat wait() until player.Character ~= nil
local char = player.Character
local perInvis = 10

Making the "Briefcase" for the player:

--Making a "briefcase"
local playerBriefCase = Instance.new("Folder", game.ReplicatedStorage)
playerBriefCase.Name = player.Name

Looping through body Parts and making them invisible:

for i,v in pairs(char:GetChildren()) do
	if v:IsA("Part") then --Making any "Part" invisible such as the "head"
		v.Transparency = 1
	end
	if v:IsA("MeshPart") then --If the user is using any kind of a roblox package
		v.Transparency = 1
	end
	if v:IsA("Accessory") then --If the user has any hats or accessories
		v.Parent = playerBriefCase
	end
	for i,x in pairs(char.Head:GetChildren()) do --To hide the face of the player
		if x:IsA("Decal") then
			if x.Name == "face" then
				x.Parent = playerBriefCase --Parenting the face in the player's "Briefcase"
			end
		end
	end
end

Delaying:

wait(perInvis)	 --Delay between powers

Making Body Parts Visible and retrieving the accessories:

for _,Folder in pairs(game.ReplicatedStorage:GetChildren()) do --Getting the user's private folder
	if Folder.Name == player.Name then
		local briefCase = Folder
		for _,child in pairs(briefCase:GetChildren()) do --Retrieving the user's items
			if child:IsA("Decal") then
				if child.Name == "face" then
					child.Parent = char.Head
				end
			end
			if child:IsA("Accessory") then
				child.Parent = char
			end
		end
	end
end
for i,v in pairs(char:GetChildren()) do --Making user's body parts visible
	if v:IsA("Part") then
		if v.Name ~= "HumanoidRootPart" then
			v.Transparency = 0
		end
	end
	if v:IsA("MeshPart") then
		v.Transparency = 0
	end
end

Full Overview:

• Getting Variables

  • Getting Player then Getting Character
  • Adding a variable for the time between invis/vis
  • Making a "BriefCase" for the user to store his/her accessories

• Looping through the body parts

  • Making Body parts under the class Part Invisible
  • Making Body parts under the class MeshParts invisible
  • Parenting the player's accessories into his "BriefCase"
  • Looping through the Head's Children
    • Getting the player's Face
    • Parenting the player's Face to his/her "BriefCase"
  • Waiting 10 Seconds

• Finding the user's "BriefCase" in ReplicatedStorage

  • Found the user's "BriefCase"
    • Grabbing the BriefCase's Children From a for loop
    • Checking if the ClassName of objects in the folder are Decals or Accessories
    • If the Object is a Decal then parent it to the user's Head which is in the user's Character
    • If the object is a Accessory then parent it to the user's Character

• Looping through the user's Character

  • Checking if the object or body part is a Part or a MeshPart
    • if the BodyPart is a Part then making it Visible by setting the transparency to 0
    • if the BodyPart is a MeshPart then making it Visible by setting the transparency to 0

Hope this helped. Sorry for any mistakes if there are, i was writing these while being sick.