liquid error: Unknown tag 'seo' included

[estelll]

jekyll 3.7.2 | Error: Liquid syntax error (dirname/_includes/head.html line 5): Unknown tag 'seo' included

I am new to jekyll and I build a site following the tutorial in https://jekyllrb.com/docs/quickstart/. It by default use a gem-based theme and it work well. And then I wanna try regular theme so I copy the directories needed(_includes, _layout,asset and so on) into my site dir. And I delete the command line of 'gem "minima", "~> 2.0"' in Gemfile and 'theme: minima' in _config.yml as described in https://jekyllrb.com/docs/themes/.
Then I ran jekyll serve but it throws an error above.

Did I miss something after delete the command lines ?

[benbalter]

Please see the usage instructions to update your Gemfile.

[stale]

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