There is a high probability that the previous xvfb process will still be running, and so xvfb will not be able to be launched, which will lead to an error.
Run ps aux | grep vfb, and if the Xvfb-related process still running, kill it before trying the operation again.
If you still do not understand the cause after checking the above, specify the -e option, such as xvfb-run -e PATH_TO_LOG_FILE ./magicpod-command-line and output the contents of the error to a file. Set the path of PATH_TO_LOG_FILE to any file for which you have write permissions.