Im Trying To Make My Music Gui Show What Is Currently Playing But Its Not Working?

The most likely reason why it doesn't work is, it only checks the TextBox once

You can transfer the line local Music = game:GetService("MarketplaceService"):GetProductInfo(script.Parent.Parent.IDhere.Text) just underneath script.Parent.MouseButton1Click:Connect(function()

This will make it, so the script will check the TextBox every time that the TextButton is clicked, this should also apply to your ID variable, as it only checks it once

Unable to cast string to int64

This means that the script attempted to give something a string, but it has expected a int64, which is a number that can go negative and positive, and up to the number

2 ^ 64 - 1

To fix this you'd want a number input, not a string input, meaning if a sound id is 123, you'd want to put in the number 123, not the string "123" or the string "rbxassetid://123", since the most likely thing you are experiencing is the string "123", you'd want to put a tonumber to transform a string to a number, therefore your code shall be:

script.Parent.MouseButton1Click:Connect(function()
local Music = game:GetService("MarketplaceService"):GetProductInfo(tonumber(script.Parent.Parent.IDhere.Text))
local ID = script.Parent.Parent.IDhere
    if string.len(ID.Text) > 0 then
        script.Parent.Text = '...'
        wait(1)
        script.Parent.Text = 'Playing: ' ..Music
    else
        script.Parent.Text = 'Error: No ID Found'
    end
end)