Hello sir_Paq -
eBay no longer allows the Out of Stock option on a listing by listing basis, so our listing creation page was updated to match their policy. You should be able to set it globally in your eBay account preferences and can ensure that your Auctiva account is updated using the "Update eBay preferences" button on the eBay Account Information page of your Auctiva account. This ensures that the options available in your account will correspond to your eBay selections, but please keep in mind that the option for selecting this on a listing by listing basis is no longer offered.
If you find a discrepancy on the listing creation page of your Auctiva account compared to the Sell Your Item form on eBay for the same category, please let us know the details of what you encounter so we can look into it.