"make the humanoid jump again after pressing spacebar twice with custom animation"

-- local script 
local player = game:GetService("Players").LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
local humanoid = character:WaitForChild("Humanoid")
local UserInputService = game:GetService("UserInputService")

local jumpCount = 0
local canDoubleJump = true

local function onJumpRequest()
	if jumpCount == 0 then
		if humanoid:GetState() == Enum.HumanoidStateType.Landed then
			humanoid:ChangeState(Enum.HumanoidStateType.Jumping)
			jumpCount = 1
		end
	elseif jumpCount == 1 and canDoubleJump then
		canDoubleJump = false
		humanoid:ChangeState(Enum.HumanoidStateType.Jumping)
		jumpCount = 2
		-- Play custom animation
		local animation = Instance.new("Animation")
		animation.AnimationId = "rbxassetid://YourAnimationIDHere"
		local animTrack = humanoid:LoadAnimation(animation)
		animTrack:Play()
	end
end

local function onHumanoidStateChange(oldState, newState)
	if newState == Enum.HumanoidStateType.Landed then
		jumpCount = 0
		canDoubleJump = true
	end
end

UserInputService.JumpRequest:Connect(onJumpRequest)
humanoid.StateChanged:Connect(onHumanoidStateChange)