Change alpha value to transparent when colliding (Lerp, DOTween)

Hey guys!
I have a little game where my player gets hit by an enemy and after each hit I want to change his alpha value a little bit until he almost becomes transparent (Color lerp). I saw that it is possible by doing a color lerp or with the DOTween plugin. Sadly I didn't manage to do it. That is what I have now:

Code (CSharp):

1. private void Fade(float endValue, float fadeDuration, TweenCallback onEnd)
2.         {
3.             if(fadeTween != null)
4.             {
5.                 fadeTween.Kill(false);
6.             }
7.  
8.             fadeTween = canvasGroup.DOFade(endValue, fadeDuration);
9.             fadeTween.onComplete += onEnd;
10.         }
11.  
12.         private void FadeOut(float fadeDuration)
13.         {
14.             Fade(0f, fadeDuration, () =>
15.             {
16.                 canvasGroup.interactable = false;
17.                 canvasGroup.blocksRaycasts = false;
18.             });
19.         }

Nothing happens to my player. Is something wrong in my code or is there another solution? I would be glad if someone can help me out