idk why this script wont work its not disabled, trying to make some thing diaper how do i fix?

Im not 100% sure what your wanting but this is what I pieced together. (assuming you are wanting this script to affect a player when they touch the part)

local tri = nil
function onTouch(part) 
    local humanoid = part.Parent:FindFirstChild("Humanoid") 
    if (humanoid ~= nil) then   -- if a humanoid exists, then
        --humanoid.Health = 0   -- damage the humanoid
    end 
    if humanoid then
        tri = true --switched to true
    else
        tri = false --switched to false
    end

    if tri == true then
    part.Transparency = 1
        part.CanCollide = false
    end

    wait(3)
    part.Transparency = 0
        part.CanCollide = true


end
script.Parent.Touched:connect(onTouch)

Your problem was where you set tri equal to true or false, based on what I think your wanting, you mixed the two up. As shown above I flipped the true and false and it worked(shown with comments.) The way you had it set was, if the part was NOT a player then set the transparency = 1.

Personally I would do it this way. Instead of setting a variable equal to true or false based on whether or not the part is a player and then checking that variable to detect if the part indeed was a player, you can skip setting a variable and just run the code if it detects a humanoid. The end goal is the same the code is just shorter.

local tri = nil
function onTouch(part) 
    local humanoid = part.Parent:FindFirstChild("Humanoid")  
    if humanoid then --checks if part is a player, if not do nothing
        part.Transparency = 1
        part.CanCollide = false
        wait(3)
        part.Transparency = 0
        part.CanCollide = true
    end
end
script.Parent.Touched:connect(onTouch)