Solved Why does this runnable not work?

To cancel it inside itself you need to use the BukkitRunnable. Otherwise you have to store the Scheduler Process ID in an Integer and cancel it by calling Bukkit.getScheduler().cancel(pScheduler);

 

Try it without the scheduler like this:

Code (Java):

new BukkitRunnable(){

     @Override
      public void run() {
           Bukkit.getConsoleSender().sendMessage(" running");
           cancel();
      }
}.runTaskTimerAsynchronously(plugin, 20, 20);

 

First off that method is deprecated use this
https://hub.spigotmc.org/javadocs/spigot/org/bukkit/scheduler/BukkitRunnable.html#runTaskTimer(org.bukkit.plugin.Plugin, long, long)

Second thing it clearly states
Asynchronous tasks should never access any API in Bukkit. Great care should be taken to assure the thread-safety of asynchronous tasks.

And there you are using Bukkit.broascastMessage. Use the link for the right thing