Fabric Language Kotlin and Yet Another Config Lib can appear together in the same modpack, but they solve different problems. Treating them as substitutes leads to missing-dependency errors.
Fabric Language Kotlin is language infrastructure
Fabric Language Kotlin enables Kotlin-specific entrypoints and bundles the Kotlin standard library plus common kotlinx libraries. It exists so Kotlin-based Fabric mods can run with a shared language layer.
Yet Another Config Lib is configuration UI infrastructure
Yet Another Config Lib, commonly shortened to YACL, is a separate library used by mods that want structured configuration screens and related configuration features. It is not a replacement for Kotlin language support.
Why a mod may require both
A mod can be written in Kotlin and also use YACL for its settings interface. In that case the mod metadata can require both libraries. Removing either one may stop the mod from loading or disable expected configuration features.
Troubleshoot each dependency separately
When Fabric Loader reports a missing dependency, follow the exact mod ID in the message. Install or update only the required library and keep its version compatible with the dependent mod.
Do not assume that installing one popular library satisfies every dependency in a Fabric pack.