I am getting "Stream specifier ' ' in filtergraph description" in ffmpeg
Your command is missing the -i token for the image input, so it should be

ffmpeg -loop 1 -framerate 24 -t 10 -i image.jpg -f lavfi ...


However, for the actual task at hand, the concat filter is not required.

ffmpeg -loop 1 -framerate 24 -t 10 -i image.jpg -f lavfi -t 10 -i anullsrc out.mp4