java.lang.NoSuchMethodError Minecraft mod

What this actually means

A mod is calling a method or field that doesn't exist in the version of the thing it's calling. It was built against a different version of Minecraft or of a library/API mod.

How to fix it

Identify the two mods involved from the signature and the stack trace. Update BOTH to builds made for the same Minecraft version. Library mods (Architectury, Kotlin, Resourceful, Bookshelf, Patchouli) are the usual half of this pair -- an outdated library breaks every mod that uses it.

What it looks like in your log

Search your crash report or latest.log for any of these:

java.lang.NoSuchMethodError:
java.lang.NoSuchFieldError:
AbstractMethodError

The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.