Why is this random 1 person team selector not working?

Firstly, GetPlayers is literally the name of a method of Players. game.Players:GetPlayers(). Use that instead. Secondly, FindFirstChild takes the name of a string. The player parameter is already the player, not only would FindFirstChild(player) not work, but FindFirstChild(player.Name) and player are the same thing, so use the latter.

And, I'm pretty sure that the previous answers should work, but if not then here you go:

TeamName = "Monster"
players = {}

function TeamPlayer(player)
    local team = game.Teams:FindFirstChild(TeamName)

    if player~= nil then
        player.TeamColor = team.TeamColor
    end
end

function SelectPlayer()
    local plrs = game.Players:GetPlayers()
    if #plrs < 1 then return end
    local selected = plrs[math.random(1, #plrs)]
    if selected then
        print('Selected player was: ' ..selected.Name)
        TeamPlayer(selected)
    end
end

SelectPlayer()

local teamName = "Monster"
local plrs = game.Players:GetPlayers()
if #plrs > 0 then
    local chosenPlr = plrs[math.random(1,#plrs)]
    chosenPlr.Team = game:GetService("Teams")[teamName]
end

Here you go, I added the line:

repeat wait() until #game.Players:GetPlayers() >= 1

this waits until there is an actual player in the game to run this function on. I created a function to encompass this scope so none of the code runs unless there is one or more players in the game.

TeamName = "Monster"
players = {}
selected = ""
function setRandomTeam()
    repeat wait() until #game.Players:GetPlayers() >= 1
    function GetPlayers()
        for _,v in pairs(game.Players:GetChildren()) do
            table.insert(players,v)
        end
    end

    GetPlayers()

    function SelectPlayer()
        local num = math.random(1, #players)
        selected = players[num]
        print('Selected player was: ' ..selected)
    end

    SelectPlayer()

    function TeamPlayer(player)
        local play = game.Players:FindFirstChild(player)
        local team = game.Teams:FindFirstChild(TeamName)

        if play~= nil then
            play.TeamColor = team.TeamColor
        end
    end

    TeamPlayer(selected)
end
setRandomTeam()

Still need help!