I tried to make a certain face decal for every player who joins my game and it did not work, why?

You need Parentheses to close the anonymous function.

Also, Why did you go through the process of finding the player in players when the PlayerAdded function gives you the player from the start?

I'm going to be using the Players service because it would make the code much more simple.

Your fixed, and simplified, code should look like this,

game.Players.PlayerAdded:connect(function(plr)
    plr.CanLoadCharacterApperence = false
    plr.Head.face.Texture = "rbxassetid//156071939"
end)-- Added Parentheses

Head is a child of the character, not the player.

To get the character, I'm going to use the CharacterAdded Function. I'm also going to use WaitForChild because it's cool to do that.

game.Players.PlayerAdded:connect(function(plr)
    plr.CanLoadCharacterApperence = false
    plr.CharacterAdded:connect(function(char)
        char:WaitForChild("Head"):WaitForChild("face").Texture = "rbxassetid//156071939"
    end)
end)

You spelled Appearance wrong.

Final Script,

game.Players.PlayerAdded:connect(function(plr)
    plr.CanLoadCharacterAppearance = false
    plr.CharacterAdded:connect(function(char)
        char:WaitForChild("Head"):WaitForChild("face").Texture = "rbxassetid//156071939"
    end)
end)

Side note,

You should really learn what error messages mean, and how to read them.

I hope I helped!

Good Luck!

If I did help, remember to accept my answer. Accepting answers gives us both reputation!