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Java 9 is not currently supported, you'll need to use Java 8.

Also read through the docs page if you haven't already.

That tutorial isn't really doing things quite right here. Have two seperate constructors for your packet like so: public PacketSendKey() {} public PacketSendKey(BlockPos blockPos) { this.blockPos = blockPos; } Then when you go to send this packet, get the mousever position from the client and pass it to the packet constructor yourself. You can't reference client-sided code in the packet constructor as the packet also needs to be constructed on the server (so it can be passed to your packet handler).

Your post is unreadable on the dark theme, you should use the Tx button to remove the formatting from it. You need to tell ForgeGradle to reobfuscate the shadow JAR, you can see how I do this here.

I think the problem could be in your code. I don't do my registration from events, rather I just call ForgeRegistries.BLOCKS.register(...) for blocks and ForgeRegistries.ITEMS.register(...) for items. You can look at my class here if you wish: https://github.com/WaffleMan0310/AncientMagicks/blob/master/src/main/java/com/waffleman0310/ancientmagicks/init/Blocks.java