Selecting random players gives error?

Here's the script:

local repstorage = game:GetService("ReplicatedStorage")
local players = game:GetService("Players")

function pickPLR()
	local plrlist = players:GetChildren()
	local selectedIndex = math.random(1,#plrlist)
	local target = CFrame.new(158.5, 44.5, 77.5)
    selectedIndex.Character.HumanoidRootPart.CFrame = target
end

while true do
	wait(5)
	pickPLR()
	wait(50)
end

Now when I test this it tells me "Attempt to index local 'selectedIndex' (a number value)"

Can anyone tell me what i'm doing wrong? The point of the script is to select a random player and teleport only that player to the target area.

You're trying to index a numbervalue, like it says. Not a player.

Use :GetPlayers() to create a table of all the players in your game, and then math.random that, like so.

function pickRandomPlayer()
	local Players = game:GetService('Players'):GetPlayers()
	local SELECTED_PERSON = Players[math.random(1, #Players)]
	local TARGET = CFrame.new(158.5, 45.5, 77.5) --Note, don't spawn the player at the part's exact Y position, set it a few studs over. It'll get them stuck very often.
	SELECTED_PERSON.Character:WaitForChild('HumanoidRootPart').CFrame = TARGET
	return SELECTED_PERSON
end

while wait(5) do
	pickRandomPlayer()
	wait(50)
end

The reason this works is because GetPlayers, GetChildren, and GetDescendants return a table. Then, you can pick a random index out of all the players by picking randomly between 1, the first player, and #Players, the length of the table.