the plugin doesn't work on 1.21
Tested on Minecraft Version 1.21 | Plugin Version 1.9.0 and 1.10.0 | Software Paper

It seems 1.21 brings some breaking changes regarding entity names. TNT (PRIMED_TNT), TNT minecarts (MINECART_TNT) and ender crystals (ENDER_CRYSTAL) don't work properly.

As a workaround: use TNT, TNT_MINECART and END_CRYSTAL in the CustomEntity section.