One of the main improvements brought about in 1.13 was mojang's creation of the Brigadier command library, it allows advanced tab completion for commands, and much a better all around command creation experience. In 1.12.2 after wanting proper command completion we introduced the system we're using currently.

With the new command library it's time for us to phase out and remove our old command library and move over to the Brigadier library.

I have outdated Information, the first comment on this issue is all that needs fixing!

The reason that this issue is marked as [Bug] as well, is because our current library is no longer working properly in it's current state, and while we could easily fix it, we should instead move over to the new Brigadier library,

Here's the error/bug with our current library:

[01:51:29] [Netty Server IO #1/ERROR] [minecraft/ArgumentTypes]: Could not serialize net.minecraftforge.server.command.ModIdArgument@71b015ed (class net.minecraftforge.server.command.ModIdArgument) - will not be sent to client!
[01:51:29] [Netty Server IO #1/ERROR] [minecraft/ArgumentTypes]: Could not serialize net.minecraftforge.server.command.EnumArgument@1fe5aa (class net.minecraftforge.server.command.EnumArgument) - will not be sent to client!
[01:51:29] [Netty Server IO #1/ERROR] [minecraft/ArgumentTypes]: Could not serialize com.ldtteam.structurize.commands.arguments.MultipleStringArgument@1be04472 (class com.ldtteam.structurize.commands.arguments.MultipleStringArgument) - will not be sent to client!

https://github.com/MinecraftForge/MinecraftForge/blob/1.14.x/src/main/java/net/minecraftforge/common/ForgeMod.java#L131

@Asherslab what's missing here?

@Raycoms i believe a PR might have been merged to fix this.
@someaddons ?

It is caused by structurize's custom argument type, which can't be deserialized client sided, but thats not an issue afaik. At least forge does say so aswell as Nightenom linked above