Given following log snippets, their explanation, and nothing else, explain what failure, if any, occurred during build of this package.

  Snippets are in a format of [X] : [Y], where [X] is a log snippet, and [Y] is the explanation.

  Snippets are delimited with '================'.

  Drawing on information from all snippets, provide a concise explanation of the issue and recommend a solution.

  Explanation of the issue, and recommended solution, should take a handful of sentences.

  Snippets:

  [HTML-Proofer found 2 failures!]: [It shows HTML-proofer ended with 2 errors.]
=============
[- ./_site/explainery/elektrifikace-dopravy.html

  *  Build error (line 370)

     No item with slug 57-doprava-ve-meste]: [It shows there was an error while building file "./_site/explainery/elektrifikace-dopravy.html" on line 370.]
=============
[- ./_site/temata/energetika.html

  *  Build error (line 778)

     No item with slug 57-doprava-ve-meste]: [It shows there was an error while building file "./_site/temata/energetika.html" on line 778.]

Issue: There were two HTML-proofer errors: Files "./_site/explainery/elektrifikace-dopravy.html" and "./_site/temata/energetika.html" are referencing the item with slug "57-doprava-ve-meste". However, no such item exists.
Resolution: Either add an item with the slug "57-doprava-ve-meste" or adjust the slug referenced from files "./_site/explainery/elektrifikace-dopravy.html" (line 370) and "./_site/temata/energetika.html" (line 778) to an existing one.
Check for typos in the slug ID.