You can compile FastAsyncWorldEdit as long as you have some version of Java greater than or equal to 16 installed. Gradle will download JDK 16 specifically if needed,
Note that if you have JRE 8 installed, Gradle will currently attempt to use that to compile, which will not work. It is easiest to uninstall JRE 8 and replace it with JDK 16.
FastAsyncWorldEdit uses NMS (net.minecraft.server) code in a variety of spots. NMS is not distributed via maven and therefore FastAsyncWorldEdit may not build without errors if you didn't install it into your local repository beforehand.
You can do that by either running Spigot's [BuildTools](https://www.spigotmc.org/wiki/buildtools/) targeting the versions needed or using Paper's [paperclip](https://papermc.io/downloads) with `java -Dpaperclip.install=true -jar paperclip.jar`.