Hi OseongKwon ,
You're correct — the OAK-D SR only has two colour cameras connected to CAM_B and CAM_C, and does not have a CAM_A socket. That's why the example using CameraBoardSocket.CAM_A doesn't work — it's trying to use a camera that doesn't exist on your device.
Most of our RGB-Depth alignment examples are written for 3-camera devices, which have a dedicated RGB camera on CAM_A and two mono/stereo cameras.
For OAK-D SR, the stereo pair consists of two colour sensors, and the correct approach is slightly different. This example is designed specifically for OAK-D SR and shows how to set up stereo depth with the right camera configuration:
luxonis/depthai-pythonblob/main/examples/StereoDepth/depth_preview_sr.py - should give you some ideas on how to use the camera.
Let me know if you have any further questions.
Thanks,
Oskar