Why it said unknown command?

i was about to download a jekyll theme, and write the command that author wrote,why it said unknown command?

I don’t think running that is a valid command… where did you see that it was? what were you trying to do? install the theme?

Posting the actual text is better than a screenshot, easier for someone to search for something.

Because you missed the command to give to the gem command, which in your case is install:

gem install 'dev-portfolio-blog'

If you want to add the gem to your project you can use bundler:

bundle add 'dev-portfolio-blog' --group jekyll_plugins