Yarn, the mappings used by the Fabric mod loader and its forks & derivatives, was originally not designed to handle Java 16.
These problems are fixed with this new build script and several other, smaller changes made.
Clone this repository and open the folder in any shell. Run ./gradlew yarn - or, if you have gradle installed, gradle yarn.
An Enigma window will open.
Select File > Export Sources and export the source code, deobfuscated and decompiled.
This process will take a while.
Java 16
Gradle 7.x recommended