ReadPixels was called to read pixels from system frame buffer, while not inside drawing frame.

这个错误通常是因为尝试在渲染帧之外调用`ReadPixels`函数所导致的。Unity的渲染管线在每个帧之间进行同步，所以你必须在渲染帧时调用`ReadPixels`函数才能正确地读取像素数据。

为了解决这个问题，你可以将`ReadPixels`函数的调用移到`OnPostRender()`函数中。`OnPostRender()`函数是在每个Camera完成渲染之后调用的，因此在这里调用`ReadPixels`函数就可以保证它在渲染帧时被正确调用。以下是修改后的示例代码：

using UnityEngine;

public class ScreenRecorder : MonoBehaviour
{
    public int captureWidth = 1920;
    public int captureHeight = 1080;
    public int captureFPS = 30;
    public string fileName = "ScreenShot";
    public bool isRecording = false;

    private int frameCount = 0;
    private Texture2D texture;

    void Start()
    {
        Screen.SetResolution(captureWidth, captureHeight, false);
        Time.captureFramerate = captureFPS;
    }

    void Update()
    {
        if (isRecording)
        {
            // Do something to start recording
        }
    }

    void OnPostRender()
    {
        if (isRecording)
        {
            string path = Application.dataPath + "/" + fileName + "_" + frameCount.ToString("0000") + ".png";
            texture = new Texture2D(captureWidth, captureHeight, TextureFormat.RGB24, false);
            texture.ReadPixels(new Rect(0, 0, captureWidth, captureHeight), 0, 0);
            texture.Apply();
            byte[] bytes = texture.EncodeToPNG();
            System.IO.File.WriteAllBytes(path, bytes);
            frameCount++;
        }
    }
}

在这个修改后的示例中，我们将`ReadPixels`函数的调用移到了`OnPostRender()`函数中，并且在`Update()`函数中只是设置了一个标志来指示是否正在录制。这样做可以确保`ReadPixels`函数只在渲染帧时被调用，避免了上述错误的发生。