I want to change sprites from SpriteRenderer while Animator is on in Unity
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Sprites cannot be modified while this animation is running.
As a beginner, you may not have the information you need to resolve it. I would appreciate it if you could let me know if there is any information that is not enough to solve.

Error message
Corresponding source code
Source code
public class FirstBall: BattleManager
public Sprite sprite_b;
public SpriteRenderer enemtrainerRend;
void Update () {
        enemtrainerRend.sprite = sprite_b;
        Debug.Log ($"Rendara 2 {enemtrainerRend.sprite.name}");
What I tried

With Animator turned off, I was able to change sprites from SpriteRenderer.
In Debug.Log in the Update () statement, the name of Sprite_b was always mentioned, but it was always the default image on the screen.

Supplementary information (FW/tool version, etc.)

The version is 2020.1.6f1.