About jekyll and github pages

Actually i tried using theme cayman for github page and now when i was trying to build it using bundle exec jekyll build , it is throwing error of gem seo-tag.
Liquid Exception: Liquid syntax error (line 6): Unknown tag ‘seo’ in /_layouts/default.html
Please help me.

I believe that theme uses the {{ seo }} tag, which requires a plugin to be installed.

If you’re not using the GitHub Pages gem to build your site locally then you’ll probably have to make sure it’s installed. Check their docs for those instructions.