It seems that there is no problem with the driver installation, and the camera has been detected normally.
From the error output of libcamera-still, it seems that the device is occupied by other software, maybe your mjpg-streamer is not closed?
You can turn off the default mjpg-streamer service and try to run libcamera-still.
Check service status:
sudo systemctl status webcamd.service
Stop the service:
sudo systemctl stop webcamd.service