how can I copy a product to another categorie

Hello,

That message indicate that the product alias is empty or contain the value that another product is using too.
Please provide steps that you made in order to have that message ; you're talking about "copying" a product but you do not provide details about it.
And please note that HikaShop product can have multiple categories, you do not need to copy a product to display it in different categories ; copying is just a way to gain time by duplicate the product content (and some fields must need to be modified afterwards, like the product code, alias...).

Regards,

Hi,

The message "Een element met dezelfde alias bestaat al" indicates that you already have a product with the same alias. So you need to check the alias of the product and fill it properly.
It has nothing to do with adding/changing the category of the product.
If you don't do any change and just click on the save button, you'll have the same error.

One more try. The only case where the message "Een element met dezelfde alias bestaat al" can display is when another product has the same alias than the product you're saving.
Again, please try to just save the product without changing the category and you should have the same error, indicating that it doesn't come from the category change but from the alias.
Note that if you have the "alias auto fill" setting activated in the HikaShop configuration and that you don't have any alias for your product, HikaShop will automatically generate it out of the product name.