The tag is present in Lintian version 2.109.6. That is the most recent version we know about.
The source of the following file is missing. Lintian checked a few possible paths to find the source, and did not find it.
Please repack your package to include the source or add it to "debian/missing-sources" directory.
Please note, that very-long-line-length-in-source-file tagged files are likely tagged source-is-missing. It is a feature not a bug.