Help Me On This SelfDestructOnDeath?

The error I found in this code:

TorsoPos = script.Parent.Torso
ExplosionDamageBlock = game.Lighting.ExplosionDamage

while wait(1) do
if script.Parent.Alien.Health < 1 then
ExplosionDamageBlock:clone().Parent = workspace -- All right, you've set that to workspace.
ExplosionDamageBlock.Position = TorsoPos.Position   -- Not all right, since you're referring back to the ExplosionDamageBlock variable (game.Lighting.ExplosionDamage), not its clone.

Perhaps use a local variable to distinguish which variable goes with which object:

ExpDmgBlck = ExplosionDamageBlock:clone()
ExpDmgBlck.Parent = workspace
ExpDmgBlck.Position = TorsoPos.Position

SIDE NOTE

What your script is doing while it is running is checking the Alien's health every second. This might affect and raise the amount of activity (lag) in a server. One of the solutions is to make the function fire when the humanoid is altered by using Changed() or Died() events:

script.Parent.Alien.Changed:connect(function (Health)
    if script.Parent.Alien.Health < 1 then
    --[[ Your function ]]
    end
  end)
  

script.Parent.Alien.Died:connect(function ()
    --[[ Your function, without the 'if' statement ]]
  end)
  

You forgot to capitalize "Clone()" and "Remove()"

TorsoPos = script.Parent.Torso
ExplosionDamageBlock = game.Lighting.ExplosionDamage

while wait(1) do
if script.Parent.Alien.Health < 1 then
ExplosionDamageBlock:Clone().Parent = workspace
ExplosionDamageBlock.Position = TorsoPos.Position
Explosion = Instance.new("Explosion")
Explosion.BlastPressure = 10
Explosion.BlastRadius = 0
Explosion.DestroyJointRadiusPercent = 0
Explosion.Position = TorsoPos.Position
Explosion.Parent = script.Parent.Torso
    wait(5)
    script.Parent:Remove()
end
end

You didn't capitalize Clone, and I wouldn't recommend you use remove.

TorsoPos = script.Parent.Torso
ExplosionDamageBlock = game.Lighting.ExplosionDamage

while wait(1) do
    if script.Parent.Alien.Humanoid.Health <= 0 then --I also see another problem, health is located inside the humanoid, not the object itself.

        ExplosionDamageBlock:Clone().Parent = game.Workspace
        ExplosionDamageBlock.Position = TorsoPos.Position
        Explosion = Instance.new("Explosion")
        Explosion.BlastPressure = 10
        Explosion.BlastRadius = 0
        Explosion.DestroyJointRadiusPercent = 0
        Explosion.Position = TorsoPos.Position
        Explosion.Parent = script.Parent.Torso
        wait(5)
        script.Parent:Destroy() --Changed to Destroy
    end
end