why I can't socket this crystal sword?

its a superior

Crystal Sword, normal White, I just can't seem to socket it with

Ral+

Amn+PAmy

superior items can not be cubed. only

Larzuk quest for them.

Also, unless you're fine with having only three OS max, don't socket that sword if it's from normal difficulty (except if it dropped in Cows). Items dropped in Normal difficulty are only capable of having 3 OS max (generalization). Items in NM can have 4 OS, and in Hell they can have up to 5 or 6 OS (generalization).

As another comment said, superior items can only get socket with

Larzuk, which means that if you want to use a superior item, it needs to drop with the exact amount of sockets you need, OR it needs to need the max socket (as

Larzuk gives you the max).

This is what makes superior items with the right amount of sockets so valuable. Because you can't even try to roll them with that recipe. Like if you want a 5 socket superior

Phase Blade, it needs to drop with 5 sockets; No other way to get it.