how come this doesn't work!?

wait(10)
game.Workspace.PlayersVeneus:Destroy()

Why does this not work?

You forgot to put '.' between Players and Veneus

wait(10)
game.Workspace.Players.Veneus:Destroy()

You forgot the period between Players and Venus.

The others are correct.

wait(10)
game.Workspace.PlayersVeneus:Destroy() --Incorrect

Is incorrect because you left out the "." which seperated Players from Veneus. The following code would work:

wait(10)
game.Workspace.Players.Veneus:Destroy() --Correct :)

Here's a tip though, let's say you want the player that you specify eariler in the script to be destroyed. You could say this.

Player = Veneus
wait(10)
game.Workspace.Players[Player]:Destroy() --Also correct.

The above example does exactly the same thing, just utilizing a new concept. You can see that the "." doesn't have to be used in this example because "Players" and "Player" are separated by brackets.

Did you wait 10 seconds before destroying that object?