No protocol specified Unable to access the X Display, is $DISPLAY set properly?

[wangyu8460958]

After I install Gooey and go to the GooeyExamples-master directory use this command:
pip install -r requirements.txt

The above commands all execute correctly.

it@it51:~/GooeyExamples-master/examples$ sudo python widget_demo.py
[sudo] password for ankon:
Writing Build Config to: gooey_config.json
No protocol specified
Unable to access the X Display, is $DISPLAY set properly?

The graphics card of my computer is Quadro P4000 , and the command "nvidia-smi" show :
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 440.31 Driver Version: 440.31 CUDA Version: 10.2 |
|-------------------------------+----------------------+----------------------+

How do I solve this problem: Unable to access the X Display, is $DISPLAY set properly?