What is the expiation for not making up the missed days from the previous Ramadan before the onset of the coming Ramadan, and for being unable to fast the coming Ramadan due to taking medication?

If you were unable to make up for missed Ramadan fasts until the next Ramadan began, and you were excused, then you are only required to make up the fasts whenever you are able, and no expiation (kaffarah) is due from you. This is because delaying the making up of fasts due to a valid excuse is permissible. As for your need for medication during the daytime in Ramadan, if it must be taken during the day and cannot be taken at night, then there is no blame upon you for taking it and breaking your fast. You should then make up the days you missed whenever it becomes easy for you to do so.