bug importing package

[shepherdpp]

I am trying to add this package into my documents but got following error message:

Extension error: Could not import extension sphinx_markdown_checkbox (exception: dlopen...

please note that the package name is with "_" instead of "-", however I tried both ways but neither worked, here's my conf.py settings:

source_parsers = {
    '.md': 'recommonmark.parser.CommonMarkParser',
}

source_suffix = ['.rst', '.md']

extensions = [
    'myst_parser',
    'sphinx.ext.duration',
    'sphinx.ext.doctest',
    'sphinx.ext.autodoc',
    'sphinx.ext.napoleon',
    'sphinx.ext.autosummary',
    'sphinx.ext.viewcode',
    'sphinx_markdown_checkbox',  # mind here I used "_" as instructed, but above message showed up
]

if I change the name to sphinx-markdown-checkbox, still error messages will show up as following:

Extension error:
Could not import extension sphinx-markdown-checkbox (exception: No module named 'sphinx-markdown-checkbox')

even though I did pip installed sphinx-markdown-checkbox instead of 'sphinx_markdown_checkbox