How to detect if a player is on a block?

You should use the Touched signal, like so:

workspace.hiPart.Touched:connect(function()
  print("hi")
end)

If you want to make this relative, use the following:

--Note that this script is a child of the part that says hi.

script.Parent.Touched:connect(function()
  print("hi")
end)

I think all of this will work. Note the Touched signal fires even if a different part touches hiPart.

If you want it to fire only when a player touches the block, use Humanoid's touched signal. I don't know how to use that signal, so that's why I used the regular BasePart's touched signal.

print ("Hi")

function onTouched(hit) 
    print("Brick Hit") 
    local human = hit.Parent:findFirstChild("Humanoid") 
    if (human ~= nil ) then 
            Brick.Transparency = 0.7 
            Brick.CanCollide = false 
            wait(4) -- this is how long the block can long
            Brick.CanCollide = true 
            Brick.Transparency = 0 
            -- a human has touched this door! 
            print("Human touched block") 
            -- test the human's name against the permission list 
        elseif (checkOkToLetIn(human.Parent.Name)) then 
            print("Human passed") 
            Brick.Transparency = 0.7
            Brick.CanCollide = false 
            wait(4) -- this is how long the block can long
            Brick.CanCollide = true 
            Brick.Transparency = 0 
        else human.Health = 0 -- delete this line of you want a non-killing block
        end 
    end 
end 

script.Parent.Touched:connect(onTouched)

Make sure to call the block "Brick" or call it however you want, but then replace the "Brick" in all these lines with what you want.