You should take a look into the action bar. It places text above the player's level and is easier to use than the boss bar.
I looked, but for plugin I need more like bossbar system with option to add percentage of that line. Maybe there is really good api & tutorial how to use it correctly?
There is no API for it, it is a hack done by spawning a ender dragon with a custom name and health and placing it in front of the player. If you want to use the boss bar you should probably use a plugin that does this for you and provides an API for it, such as https://www.spigotmc.org/resources/api-bossbarapi-1-7-1-8-1-9-1-10.7504/.
Already tryed this, but I managed to get errors a lot wh3n creating boss bards. Is there maybe tutorial for this? Video?
I wouldn't know. Are you sure the plugin supports 1.11? Post the errors you recieve in the discussion of the plugin so the author can help you.
That's a shame, in 1.9+ bossbar api was added to make our lives easier. Show us your code and the errors so we can help you out.
