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