Peace be upon you. According to Grand Ayatollah Khamenei (may Allah protect him): If someone has a missed fast from Ramadan, they cannot perform recommended fasting. However, fasting on behalf of another person, whether for a fee or voluntarily, is permissible. According to Grand Ayatollah Sistani (may Allah protect him): It is not allowed to voluntarily fast, even if it is a vow, for someone who has a missed fast from Ramadan. However, if they forget or are unaware that they have a missed fast and fast voluntarily, their fast is valid once they remember or learn of their obligation after completing it. It is permissible to fast voluntarily for someone who has a mandatory fast due to expiation, a vowed fast, or similar obligations. Additionally, one can perform the obligatory fast on behalf of another person even if they themselves have a missed fast from Ramadan. May you be successful in all good deeds.