Re:Sell is a feature that Skiddle offer to all event organisers, permitting 2 criteria are met:

You will only be able to request the resale of your ticket(s) if the ticket type you have purchased has sold out. This does not include 'currently unavailable' or 'not on sale'.

If the ticket type has sold out and your order is not eligible, this means the promoter has not enabled Re:Sell for the event.

You can still pass on your ticket(s) privately to avoid losing out on the money spent. Head here for help with transferring the order over to the new recipient.