Can someone help fix this script that makes part explode on touch?

You never defined "part", yet you used it. Instead, continue using script.Parent. That would make the script look like this:

debounce = true

script.Parent.Touched:connect(function(hit)
    humanoid = hit.Parent:FindFirstChild("Humanoid")
    if humanoid ~= nil and debounce == true then
        debounce = false
        player = game.Players:GetPlayerFromCharacter(hit.Parent)
        script.Parent.BrickColor = BrickColor.new("Really red")
        if script.Parent.BrickColor == BrickColor("Really red") then
            explosion = Instance.new("Explosion")
        explosion.Position = script.Parent.Position
            explosion.Parent = game.Workspace
            script.Parent.BrickColor = BrickColor.new("Institutional white")
            debounce = true
        end
        end
end)

or you could simply put another variable for part at the top to make it look like this (this would be the better solution):

debounce = true
part = script.Parent

part.Touched:connect(function(hit)
    humanoid = hit.Parent:FindFirstChild("Humanoid")
    if humanoid ~= nil and debounce == true then
        debounce = false
        player = game.Players:GetPlayerFromCharacter(hit.Parent)
        part.BrickColor = BrickColor.new("Really red")
        if script.Parent.BrickColor == BrickColor("Really red") then
            explosion = Instance.new("Explosion")
        explosion.Position = part.Position
            explosion.Parent = game.Workspace
            part.BrickColor = BrickColor.new("Institutional white")
            debounce = true
        end
        end
end)

The last thing that I noticed (which isn't an error, rather an unnecessary line of code), is that you defined the characters player, which you aren't using. If you aren't going to use this later on in the code, I suggest getting rid of it. Hope I helped.

-Cmgtotalyawesome

Script without debounce:

local part = script.Parent
part.Touched:connect(function()
    local Boom = Instance.new("Explosion", part)
---Add more properties for Boom---
end

In your script you said "Part" even though it doesn't exist in your script, the error you got was "Attempt to index a global "Part"" or something like that, make the script simple btw because you don't need debounce nor that other strange stuff you added in your code

script with debounce:

local part = script.Parent
local debounce = true
local waitTime = 2 -- Change to how long it takes the explode when touched again
part.Touched:connect(function()
if debounce == true then
    local Boom = Instance.new("Explosion", part)
    --Add more properties for Boom
    debounce = false
else
    wait(waitTime)
    debounce = true
end
end