Video Encoded using FFmpeg is not being played in Azure Kinect Viewer

I captured a video using a Kinect Azure DK device and could play it back using Azure Kinect Viewer (v.1.4.1). However, when I encode a video using FFmpeg, the command is given below. I can not playback the video using Azure Kinect Viewer. I get a message:

Failed to open Recording

ffmpeg -i .\RGB.mkv -c:v libx264 -preset veryfast -crf 18 -c:a aac -b:a 128k RGG_Enc.mkv

I tried with different codecs and different settings, but could not play the video in Azure Kinect Viewer. Can someone suggest to me what to do?

This video contains only one channel (RGB) from the original video. I am able to play this video using Azure Kinect Viewer.