Asciidoctor-Bibtex can't find .bib file?

Do this in your site config:

[markup.asciidocExt]
  extensions = ['asciidoctor-bibtex']
  workingFolderCurrent = true

See https://gohugo.io/configuration/markup/#workingfoldercurrent.

That will allow you to use this structure, where the .bib file is adjacent to the .adoc file.

content/
├── examples/
│   ├── example-1/
│   │   ├── index.adoc
│   │   └── references-1.bib
│   ├── example-2.adoc
│   └── references-2.bib
└── _index.md

You might also want to do this:

[[module.mounts]]
source = 'content'
target = 'content'
excludeFiles = '**.bib' # do not publish .bib files when building the site

See https://discourse.gohugo.io/t/using-bibtex-files-with-asciidoc-content/56119.