Hi,
Well, as the error message implies, please make sure that the alias that you're using for your categories is not already set for another category you already have.
If you don't enter any alias and have the "Alias automatic filling" setting activated in the HikaShop configuration, the system will try to use the category name if you don't enter any alias. So if you have another category with an alias corresponding to the name of your category and you don't enter an alias, then you'll also get that error message.