Hey, my english is a little rubbish but i hope you can understand. I've downloaded the firstaid 1.8.2 for 1.15.2 to use in a server but the health bar is not appearing, only the hub. Is there anything wrong with compatibility or I need to change it in the server?

You need the mod on the server and the client. The healthbar being absent is intentional, you can reenable it in the first aid config if you want to. But there are better indicators in FirstAid, like the FirstAid Overlay HUD