GTC duration is actually unlimited duration : listing will be active until you deactivate it. This option is usually preferred, because it maintains listing seniority. For more information on listing duration, please refer to eBay Help page on listing duration.
Checkbox “Automatic relist” is usefull for other listing duration than GTC. It will relist automatically all PrestaShop products after listing duration expired.
Caution : GTC & 30 days durations are not always available. In France, these 2 options are only available for seller with an active eBay shop.
Possibles errors :
GTC duration can be refused, even for seller with an active shop. There can be multiple reasons linked to eBay, including some categories which does not allow GTC duration. In this case, please contact eBay merchant support, or reduce listing duration.