How can i cast one HUD to every character of my game??

Made a simple hud with health bar and inventory but its only working for one character

You should be able to add a HUD for every character with:

Just connect “Event BeginPlay” with “Create Widget” and then your “Viewport”. Choose the class (your UserInterface) you created to be your HUD. You can put this in your Character Blueprint, if only the same characters should use the blueprint or you put it into your GameMode Blueprint so that everybody has it.