Why does the read the docs service not build after cloning a GitHub repository?

Question

Documentation

Tried to start writing documentation. Syntax .mdOn a local machine using mkdocs, everything works fine, I write mkdocs servethat the server starts up with documentation displayed.

The picture shows the location of the documentation: folder - /docs. The configuration file mkdocs.yml for mkdocs is at the root of the project.
I want to use the Read the Docs service for documentation hosting, but when I try to build, an error appears:

At the same time, if I try to run the command mkdocs build --cleanon a local machine, the site folder with the documentation is created without problems in the root of the project.
Full error log:

WARNING -  Config value: 'nav'. Warning: Unrecognised configuration name: nav 
INFO    -  Cleaning site directory 
INFO    -  Building documentation to directory: /home/docs/checkouts/readthedocs.org/user_builds/ray62/checkouts/latest/_build/html 
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/mkdocs/nav.py", line 337, in _follow
    raise StopIteration
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/bin/mkdocs", line 10, in <module>
    sys.exit(cli())
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/mkdocs/__main__.py", line 156, in build_command
    ), dirty=not clean)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/mkdocs/commands/build.py", line 282, in build
    build_pages(config, dirty=dirty)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/mkdocs/commands/build.py", line 213, in build_pages
    site_navigation = nav.SiteNavigation(config)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/mkdocs/nav.py", line 44, in __init__
    config, self.url_context)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ray62/envs/latest/lib/python3.7/site-packages/mkdocs/nav.py", line 392, in _generate_site_navigation
    config_line, url_context, config):
RuntimeError: generator raised StopIteration

Tell me how to host the documentation on Read the Docs?
UPD: Problem solved by changing python version from 3 to 2 in admin read the docs