Converting ipynb file as markdown

Hi everyone,

I have been trying to download an ipynb file from Jupyter as a markdown file. First, I tried doing it through the web but it gave me the following error:

Then I tried through the command prompt with the following command:
jupyter nbconvert --to markdown filename.ipynb
but have had no luck. It gave me the following error.
‘jupyter’ is not recognized as an internal or external command, operable program or batch file.

Any help would be appreciated. Thank you!