Help with code that chooses a player at random when there are at least 2 players in the game?

Indenting is very, very important. Makes it really easy to spot your if statement that was out of place

dft = {} 
local plrs = #game:GetService('Players'):GetChildren()

function GetPlayers()
    local c = game.Players:GetChildren() 
    for i = 1, #c do 
        table.insert(dft, c[i].Name) 
    end 
end

--if plrs >= 1 then -- This shouldn't be here

function Randomize()
    GetPlayers()
    local d = math.random(1, #dft) 
    local s = d 
    local m = Instance.new("Message", game.Workspace) 
    m.Text = "The random player is ... "..dft[s].."!" 
    wait(2) 
    m:Remove()
    local rndm = game.Players:FindFirstChild(dft[s])
    if (rndm ~= nil) then 
        local Player = rndm.Character
        Player:MoveTo(Vector3.new(6, 13, 49))
    end
end

while wait(5) do    
    print("Done waiting.")
    if plrs >= 1 then -- If statement goes here
        print("Randomizing.")
        Randomize()
    else
        print("Not enough players.")
    end
    -- Reassign variable with the new current amount of players
    plrs = #game:GetService('Players'):GetChildren()
end

Pretty easy, you get the children of players and get a random value out of the table like this

while wait() do
    local plrs = #game.Players:GetPlayers()
    if plrs >= 2 then
        local randomPlayer = game.Players:GetPlayers()[math.random(1, plrs)]
    end
end

You would add a sort of debounce to the script though.

(I didn't fix your script because this code would probably be way better)

But the problem to your script is most likely because you don't put the #players in the loop wich means it will only get it once, at the start of the script.