Hawzah News Agency – Is it necessary to perform last year's qadha fast before the beginning of this year's Ramadhan or can I keep it later?
Answer: It is necessary to observe the Qadha of the previous year's fast before the next Ramadhan but if you have failed to do so, you should observe the Qadha in the following year(s) and you should also give kaffara of 750 grams of food stuffs for each day to the poor. If the Qadha is delayed again in the following years, the kaffara is not repeated.
Source: https://www.sistani.org/