How To Use Tables To Assign A NPC A Shirt?

Firstly, if you care about performance at all, do not assign the parent in the Instance.new("Shirt", npc) call. It's better to use local shirt = Instance.new("Shirt") then assign the parent afterwards: shirt.Parent = npc. You can read more about that here if you like.

Anyways, the problem here is that you are assigning the shirt to the wrong thing. On line 15 you wrote: selectedNPC = NPCs[NPCNum] which would return either 'MemeGuy1', 'Customer1' or 'BusinessMan'. Note how they are all string objects. Then, you are trying to index the first element of that string, thinking it is actually a table.

In order to solve this, you would need to put your variables MemeGuy1, etc. into a table, then index that table. For example:

local npc = script.Parent

-- no need to assign each one to a table of length one, instead just assign it to the string object
MemeGuy1 = 'rbxassetid://...'
Customer1 = '...' 
BusinessMan = '...' -- fill these with their respective rbxassetids

NPCs = {MemeGuy1, Customer1, BusinessMan} -- putting variables instead of strings that match variable names

math.randomseed(tick())
NPCNum = math.random(#NPCs)
selectedNPC = NPCs[NPCNum]

local npcShirt = Instance.new("Shirt")
npcShirt.ShirtTemplate = selectedNPC
npcShirt.Parent = npc

This should fix the issue.

EDIT:

Another possible solution - one that I would personally choose - is creating a table of tables. That way, you will be able to select a random NPC and still be able to define properties to each NPC such as name, shirtTexture, etc. This would allow your code to be easily upgrade-able if you needed to, for example, add an Age property to every NPC.

Here's what I mean:

local NPCs = {
	{
		NPCName = 'MemeGuy1',
		ShirtTexture = 'rbxassetid://...'
	},
	{
		NPCName = 'Customer1',
		ShirtTexture = 'rbxassetid://...'
	}
}

math.randomseed(tick())

selectedNPC = math.random(#NPCs)
local npcShirt = Instance.new("Shirt")
npcShirt.ShirtTemplate = NPCs[selectedNPC].ShirtTexture
npcShirt.Parent = npc