OpenEdx Exams - System Check don't work

Hey,

I’m working with Proctored Exam, im following step in:
https://support.edx.org/hc/en-us/articles/360000219147-Setting-up-the-proctoring-software.
Im stuck at step 2, when i click button “Start System Check”, im redirect to:
http://{myhost}:18000/courses/course-v1:edx+P4E2020+20202/courseware/83de68050669417c96bc3eb0234b1da0/253f219b6f9849b4b3252a3b0904b32e/None
It’s 404 page and it said :“Position None is not an integer!”.
My setting:

PROCTORING_BACKENDS {'DEFAULT': 'null', 'null': {}}
PROCTORING_SETTINGS {}

Im using Docker devstack, I tried to download RPnow from website but cannot validate my course’s exam code.
Im new, hope some help.
Thanks everyone !