Developing a jakyll theme

Envato Customers Web and Code
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Hello,

I purchased this jakyll theme to deploy on github pages: Sera – One page multi-purpose Jekyll theme – Jekyll Themes

However, when I run

bundle exec jekyll serve --livereload

I get an error

Calling DidYouMean::SPELL_CHECKERS.merge!(error_name => spell_checker)' has been deprecated. Please call DidYouMean.correct_error(error_name, spell_checker)’ instead.
Configuration file: /Users/gosztola/github/agosztolai.github.io/_config.yml
Source: /Users/gosztola/github/agosztolai.github.io
Destination: /Users/gosztola/github/agosztolai.github.io/_site
Incremental build: disabled. Enable with --incremental
Generating…
Liquid Exception: undefined method tainted?' for "":String in /Users/gosztola/github/agosztolai.github.io/_layouts/post.html ------------------------------------------------ Jekyll 4.2.0 Please append –traceto theservecommand for any additional information or backtrace. ------------------------------------------------ /Users/gosztola/.gem/gems/liquid-4.0.3/lib/liquid/variable.rb:124:intaint_check’: undefined method `tainted?’ for “”:String (NoMethodError)

  return unless obj.tainted?
                   ^^^^^^^^^

Could someone help with this?