xvfb-run: error: Xvfb failed to start

[xmustu]

When I run python data/generation_script.py --rollouts 1000 --rootdir datasets/carracing --threads 8, the error below happened,

xvfb-run -s "-screen 0 1400x900x24" --server-num=1 python -m data.carracing --dir datasets/carracing/thread_0 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=2 python -m data.carracing --dir datasets/carracing/thread_1 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=3 python -m data.carracing --dir datasets/carracing/thread_2 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=4 python -m data.carracing --dir datasets/carracing/thread_3 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=5 python -m data.carracing --dir datasets/carracing/thread_4 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=6 python -m data.carracing --dir datasets/carracing/thread_5 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=7 python -m data.carracing --dir datasets/carracing/thread_6 --rollouts 126 --policy brown
xvfb-run -s "-screen 0 1400x900x24" --server-num=8 python -m data.carracing --dir datasets/carracing/thread_7 --rollouts 126 --policy brown
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start
xvfb-run: error: Xvfb failed to start

Anyone encounters the same problem?