If you received an invitation to Sell Something as part of a Special Offer promotion, generally, you need to Activate that promotion.
Also, there are surcharges to use special features. Not everything is free with a listing promo like that. You need to read the fine print. Like whether it applies to ebay.ca or ebay.com.
Also also, Final Value Fees are charged if the item sells. Those are rarely free.
More information is required in order to give you better answers than above.