Hello, guys. I'm not into technical stuff much, please, can you explain to me how to use the mod on the following example :

I want to ban item called Soulstone Empty in Custom Npcs mod.

F3 of the item :

error I get each time I'm using /ban command :

I tried giving command in lots of different ways, however, non seems to work and give the error from the above :

minecraft 1.11.2

I browsed everything I could to find more explanation how to do it, but I did not find anything more than basic instructions, written to the mod om every minecraft website.

Thank you.

All the info you need is on the CurseForge project.
https://www.curseforge.com/minecraft/mc-mods/item-blacklist

/ban isn't a valid command as ban is a sub command of /blockitem as mentioned on the CF project and in your screenshot. /blockitem ban is what you would be using instead.
Also that would overlap with the server ban....