Problem with an error: invalid argument #2 to 'random' (range is empty) ?

On line 22, the FreeCellsList table might be empty, which in turn causes #FreeCellsList to be 0. Putting 0 as the max number for the RNG to choose causes problems.

Also, for some reason getting the length of tables doesn't work when they are dictionaries (or tables that use keys to index values [e.g ["foo"] = 20]). Arrays however will work (tables that have uses numbers to index values.). This might've been the main problem you're dealing with.

Dictionary vs Array:

local dictionary = { -- [key] = <some value>
    ["foo"] = 20,
    ["bar"] = 124,

}
local array = { -- <value>,<value>,<value>
    "foo",
    "bar",
    true
{

You can stop the error by adding an if-statement, and get the actual count by looping through the dictionary.

local RemoteEvent = game.ReplicatedStorage.ChangeTeam

FreeCellsList = {
    CL1 = workspace.CellLocation1,
    CL2 = workspace.CellLocation2,
    CL3 = workspace.CellLocation3
}

local function CountTable(t) -- counts the number of items in a dictionary.
    local Total = 0
    if typeof(t) == "table" then
        for _, _ in pairs(t) do -- the key and value in the dictionary isn't needed.
            Total += 1
        end
    end
    return Total
end

RemoteEvent.OnServerEvent:Connect(function(player,teamcolor)
    player.TeamColor = teamcolor
    player:LoadCharacter()
    wait(0.1)

    --Underneath, assign a cell for each player in the team 

    local keys = {}
    for k in pairs(FreeCellsList) do 
        table.insert(keys,k)
    end

    if player.TeamColor == BrickColor.new("Deep orange") then
    local TableCount = CountTable(FreeCellsList) 
    if TableCount <= 0 then
        return -- or do something else.
    end
        local ChoiceIndex = math.random(1,TableCount)
        local ChoiceKey = keys[ChoiceIndex]
        local choice = FreeCellsList[ChoiceKey]
        table.remove(FreeCellsList,ChoiceIndex)
        player.Character.HumanoidRootPart.CFrame = CFrame.new(game.Workspace.choice.Position)
        print("Téléporté à une cellule aléatoire")
    end
end)