Can't find enough markers by the mono_camera in the velo2_gazebo

[ChadJing]

As the figure shows, I tested the calibration software on the gazebo platform which was provided as velo2cam_gazebo package. When I ran the command:
roslaunch velo2cam_calibration mono_pattern.launch camera_name:=/blackflys image_topic:=image_raw frame_name:=blackflys
it told me
"[ WARN] [1724067266.812590398, 13331.354000000]: 2 marker(s) found, 4 expected. Skipping frame..."
Anyone who have been through the same problem?