Making up Missed Days of Ramadhan

Sheikh Saalih Al-Fawzaan حفظه الله تعالى was asked:

Q: There is a woman who has many days of Ramadhan to make up. Is it allowed for her to fast the recommended six days of Shawwal before making up the missed days of Ramadhan in order to complete the six days before the month of Shawwal ends?

A: No, upon her is to complete her missed days from Ramadhan first, as the Prophet ﷺ said: "whoever fasts Ramadhan THEN follows it up with six days from Shawwal."

So (in this case) she hasn’t fasted the (entire) month of Ramadhan yet. So long as there remains missed days, she hasn’t completed the fasting of it (i.e. Ramadhan).

https://www.alfawzan.af.org.sa/ar/node/14891