Admin Script in ServerScriptService won't make player run faster?

use print, good way to debug what's wrong:

game.Players.PlayerAdded:Connect(function(Player)
  print("Player has joined the game")
  Player.Chatted:Connect(function(Message)
    print("Message out:", Message)
    if Player.UserId == 2536746511 then
      print("User is admin")
      local Arguments = string.split(Message, " ")
      local speed = tonumber(Arguments[3])
      print("Arguments:", Arguments)
      print("Speed:", speed)
      print("Command:", Arguments[1])
      if Arguments[1] == "!walkspeed" then
        print("Command is !walkspeed")
        local Target = Arguments[2]
        print("Target name:", Target)
        print("Target object:", workspace:FindFirstChild(Target))
        game.Workspace[Target].Humanoid.WalkSpeed = speed
        print("Speed of object was changed successfully")
      end
    end
  end)
end)

Out of these prints, if you:

• Don't see "Player has joined the game" then your player joins before your code connects function to PlayerAdded, to fix this, you can call the function for existing players:

local function OnPlayerAdded(Player)
    -- previous code
end

for _, Player in ipairs(game.Players:GetPlayers()) do
    task.spawn(OnPlayerAdded, Player)
end

game.Players.PlayerAdded:Connect(OnPlayerAdded)

task.spawn will make sure that the function will run independently from the loop just in case more players are in the game before PlayerAdded fires

• Don't see "User is admin" then you can be playing with a different account, but with your double ID check that won't be the case

• Don't see "Command is !walkspeed" then you type the command incorrect, there can be double space in your chat message for example

• In "Target:" target name is empty, this is same case as before, you could include one more space in the command, that will turn the Arguments table to be one field bigger and will break your script

• See "Target object: nil", this means that you wrote the name incorrectly, though since you said there are no errors this should not be the case too

• See "Speed of object was changed successfully" then make sure the speed you typed in command is not 16 or 17 or close to these numbers since then you won't see difference between new and old speed, if you see this message but the speed does not change, there can be a different script changing the speed?