What is the expiation for someone who did not pay Zakat al-Fitr this year due to not having the full amount for five people?

Zakat al-Fitr must be paid in cash when there is a predominant benefit [in doing so]. If a person only possesses what suffices for his needs, then he must pay it in the order mentioned by Ibn Qudamah: He starts with himself, then his wife, then his slaves, then his young children, then his relatives. Zakat al-Fitr remains a debt and does not lapse if not paid on time.