I found I had this issue on Windows 10 when I ran:
streamlit config show > ~/.streamlit/config.toml

For example, with no config.toml file I run:
streamlit hello

And I am greeted with:
  You can now view your Streamlit app in your browser.

  Local URL: http://localhost:8501
  Network URL: http://192.168.100.75:8501

I then run:
streamlit config show > ~/.streamlit/config.toml

And then running streamlit hello I am greeted with:
PS C:\Users\sbiggs> streamlit config show > ~/.streamlit/config.toml
PS C:\Users\sbiggs> streamlit hello
Traceback (most recent call last):
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\sbiggs\AppData\Local\Continuum\anaconda3\Scripts\streamlit.exe\__main__.py", line 4, in <module>
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\__init__.py", line 103, in <module>
    from streamlit.DeltaGenerator import DeltaGenerator as _DeltaGenerator
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\DeltaGenerator.py", line 27, in <module>
    from streamlit import caching
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\caching.py", line 40, in <module>
    from streamlit.hashing import Context
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\hashing.py", line 41, in <module>
    from streamlit.folder_black_list import FolderBlackList
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\folder_black_list.py", line 39, in <module>
    if config.get_option("global.developmentMode"):
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\config.py", line 90, in get_option
    parse_config_file()
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\config.py", line 888, in parse_config_file
    _update_config_with_toml(file_contents, filename)
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\streamlit\config.py", line 810, in _update_config_with_toml
    parsed_config_file = toml.loads(raw_toml)
  File "c:\users\sbiggs\appdata\local\continuum\anaconda3\lib\site-packages\toml\decoder.py", line 227, in loads
    original, i)
toml.decoder.TomlDecodeError: Found invalid character in key name: '#'. Try quoting the key name. (line 1 column 3 char 2)

The only way I have found to fix it, is to completely delete the file, and then write it a fresh.