Thank you everyone for the reports, if you are unable to start the Season of Witchcraft questline and have not yet beat the Vessel of Hatred main campaign, you need to complete the Vessel of Hatred Prologue before having the option to start the seasonal questline.
This is only for Vessel of Hatred owners who have not yet completed or did not choose to skip the Vessel of Hatred campaign, as both the Prologue for the campaign and Season of Witchcraft storyline begin in Gea Kul.