Praise be to Allah.
The leading scholars are unanimously agreed that the one who does not fast some days in Ramadan has to make up those days before the next Ramadan comes. They quoted as evidence for that the report narrated by al-Bukhaari (1950) and Muslim (1146) from ‘Aa’ishah (may Allah be pleased with her), who said: I would owe Ramadan fasts, and I would not be able to make them up until Sha‘baan came, because of my duties towards the Messenger of Allah (blessings and peace of Allah be upon him)
Al-Haafiz ibn Hajar said: From the fact that she was keen to do that in Sha‘baan, it may be understood that it is not permissible to delay making up missed fasts until the next Ramadan begins.
End quote from Fath al-Baari (4/191)
If making up of the fast is delayed until the next Ramadan begins, this delay must either be due to some excuse or there is no excuse for it. With regard to the one who delays it because of an excuse, there is no sin on him and he does not have to do anything other than making up the missed fasts. As for the one who delayed it with no excuse, he is sinning by delaying it, and he definitely has to make up the missed fasts, but does he also, along with making up the missed fasts, also have to feed the poor or not? This is a matter concerning which the scholars differed, but the more correct view is that he does not have to feed the poor. This has been discussed in fatwa no. 26865
Based on that, you have to make up the days on which you did not fast in Ramadan in previous years, if you are able to fast. If you are not able to fast in summer, but you are able to do so in winter, then you must fast the days that you owe in the winter.
If you are not able to fast because of sickness, and this sickness is ongoing in such a way that you will not be able to fast in the future – according to the views of a trustworthy doctor – then you do not have to fast, but you have to feed one poor person for each day that you did not fast. If you do not have any money, then this fidyah (penalty – of feeding the poor) is waived in your case, and you do not have to do anything, because Allah, may He be exalted, does not burden any soul with more than it can bear.
For more details on that, please see fatwa no. 171235
And Allah knows best.