Why would this resizing fieldofview with for loop won't work as intended?

You can simple put the zoom script inside your script to hold the key for run

Here is fixed script:

local player = game.Players.LocalPlayer
local Char = player.Character or player.CharacterAdded:Wait()
local humanoid = Char:WaitForChild("Humanoid") -- Do not use script.Parent.Humanoid, use Char:WaitForChild("Humanoid")
local UserInputService = game:GetService("UserInputService")
local Running = false

UserInputService.InputBegan:Connect(function(Input, gameProcessed)
	if UserInputService:IsKeyDown(Enum.KeyCode.LeftShift) and not gameProcessed then
        Running = true
        humanoid.WalkSpeed = 32
		-- Added the camera zoom here.
		for i = 1,5 do
			game.Workspace.CurrentCamera.FieldOfView = (70+(i*2))
			wait()
		end
    end
end)

UserInputService.InputEnded:Connect(function(Input, gameProcessed)
	if Input.KeyCode == Enum.KeyCode.LeftShift and not gameProcessed then
		Running = false
		humanoid.WalkSpeed = 16
		-- Added the camera zoom here.
		if Running == false then
			for i = 1,5 do
		        game.Workspace.CurrentCamera.FieldOfView = (80-(i*2))
		        wait()
		    end
		end
	end
end)

-- Removed the repeat wait() until ...

hope it helped :)

Doubts or errors? tell me in the comments