Is it possible to complete fasting the missed days of Ramadan after the next Ramadan, and is expiation due for them, and what is its amount?

The days you missed fasting are a debt that must be repaid, according to the verse: "And whoever is ill or on a journey - then an equal number of other days." If you are able to make them up before the next Ramadan, you are obligated to do so. Otherwise, you must make them up whenever you are able. As for the fidyah (expiatory feeding), it is not obligatory if the delay was due to a valid excuse like illness. However, if it was without a valid excuse, then you must feed one poor person for each day whose repayment was delayed until the entry of the subsequent Ramadan. The amount is a mudd of food (approximately 750 grams), but the more cautious approach is half a sa' (two mudds).