My fly up 'E' using 'Y' value script isn't working?

The title says it all:

local player = game.Players.LocalPlayer
local character = player.Character
local uis = game:GetService("UserInputService")
local mouse = player:GetMouse()
mouse.KeyDown:Connect(function(key)
	print(key)
	if key == Enum.KeyCode.E then
		character.HumanoidRootPart.Position.Y = character.HumanoidRootPart.Position.Y + 1
	
	end
end)

Thanks!

It's because you didn't use InputBegan a class of UserInputService. It fires when a player has made an interaction with their keyboard. Also mouse.KeyDown is depricated/outdated userinputservice instead

local uis = game:GetService("UserInputService")
local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
uis.InputBegan:Connect(function(input, gameProcessed)
	if input.UserInputType == Enum.UserInputType.Keyboard then
		if input.KeyCode == Enum.KeyCode.E and not gameProcessed then
 character.HumanoidRootPart.Position.Y = character.HumanoidRootPart.Position.Y + 1
				end
			end
		end
	end
end)

Hello.

Problem

You used Position instead of SetPrimaryPartCFrame().

Solution

Use SetPrimaryPartCFrame(). It sets the PrimaryPart's CFrame.

Recommendations

Use UserInputSrvice as Mouse.KeyDown is deprecated.

Fixed Code (Place it in StarterCharacterScripts)

local UserInputService = game:GetService("UserInputService")
local character = script.Parent

UserInputService.InputBegan:Connect(function(input, gameProcessedEvent)
	if input.KeyCode == Enum.KeyCode.E and not gameProcessedEvent then
		character:SetPrimaryPartCFrame(character:GetPrimaryPartCFrame() * CFrame.new(0, 1, 0))
	end
end)