Java 16 removed the ability for Forge to reflect into Java internals - meaning, it broke some of the stuff we do.
1.17 will work with Java 16 natively, but for anyone on 1.16, you need some workarounds.
Note that if you're using any version below 1.16.5-36.1.4 (that includes 1.15), this will not work. It's not possible to launch your version of the game on Java 16.
If you're on a more recent version however...
to your JVM arguments should make it work.
Ideally, however, you should just downgrade to Java 8 until 1.17 is released.