Unity Canvas - Use screen space Camera

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_12.png

Unity Screen Capture only RectTransform

using System.Collections;
using System.IO;
using UnityEngine;

namespace Utils
{
    public static class CaptureRect
    {
        public static IEnumerator Capture(RectTransform rectT, string savePath)
        {
            // your capture canvas
            var mainCanvasRect = GameObject.FindWithTag(TagHelper.MainUICanvas).GetComponent<RectTransform>();
            var camera = Camera.main.GetComponent<Camera>();
            
            yield return new WaitForEndOfFrame();
            
            var screenPos = camera.WorldToScreenPoint(rectT.gameObject.transform.position);
            var resHeight = (int)((rectT.sizeDelta.y / mainCanvasRect.sizeDelta.y) * Screen.height);
            var resWidth = (int)((rectT.sizeDelta.x / mainCanvasRect.sizeDelta.x) * Screen.width);
            var heightOffset = (int) (-resHeight * 0.5f + screenPos.y);
            var widthOffset = (int) (-resWidth * 0.5f + screenPos.x);
            RenderTexture rt = new RenderTexture(Screen.width, Screen.height, 24);
            camera.targetTexture = rt;
            Texture2D screenShot = new Texture2D(resWidth, resHeight, TextureFormat.RGB24, false);
            camera.Render();
            RenderTexture.active = rt;
            screenShot.ReadPixels(new Rect(widthOffset, heightOffset, resWidth, resHeight), 0, 0);
            camera.targetTexture = null;
            RenderTexture.active = null;
            GameObject.Destroy(rt);
            var bytes = screenShot.EncodeToPNG();
            File.WriteAllBytes(savePath, bytes);
            GameObject.Destroy(screenShot);
        }
    }
}

Change Anchors From min [0,0] max [1,1] to min [0,1] max [0,1] (Top-Left)

var size = captureRect.rect.size;
var pos = Vector2.zero;
pos.x = Mathf.FloorToInt(size.x / 2);
pos.y = -Mathf.FloorToInt(size.y / 2);
captureRect.anchorMin = Vector2.up;
captureRect.anchorMax = Vector2.up;
captureRect.anchoredPosition = pos;
captureRect.sizeDelta = size;

Error will show if content cannot display in screen.