I figured out your issue. In your Resume button, you assigned the UnPauseGame() action from the Prefab Game Manager. You should be assigning the Game Manager from the Scene instead.
Right now, your UnPauseGame() is modifying the gameIsPaused of the Prefab. That’s why you can’t unpause. The Scene Game Manager is never modified.
I don’t know is it the reason or not but i tried to assign from the scene before but didn’t work either. Then I just delete the action of the button and add it again like Mi Ni said it his topic and the function work again. Tai may tried this way or Terence way to fix it. Tks you guys alot.