...with the greatest respect to the reply, but I've already tried the one by thebigsmileXD and the bar didn't show up.
Maybe you used BossBarAPI, not BossAnnouncement? BossBarAPI is the API to create BossBars, while BossAnnouncement is basically the plugin you're looking for that creates the bossbars. Get it here: https://poggit.pmmp.io/ci/thebigsmileXD/BossAnnouncement/BossAnnouncement
Use pocketmine tools this time because the plugin's already updated to alpha7 but remember: Bumping api with this tool may damage the plugin!