What does an elder need to do if they can’t fast?

They would just need to pay the fidya because they will not be able to make up the days due to their old age.
It is the same as the fidya of a chronically ill person: the feeding of 30 poor muslims.
The evidence is in the Quran:
Allah (swt) says what could be translated as: “… compensation can be made by feeding a needy person ˹for every day not fasted˺. But whoever volunteers to give more, it is better for them…” [Al-Baqara, 2:184]

The report narrated by Abu Dawood from ibn ‘Abbas concerning the verse (interpretation of the meaning):
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day).”