Allah the Exalted has categorically made fasting obligatory upon those who are healthy and resident (i.e., not travelers) and who have no valid excuse that would prevent them from fasting. As for the sick, travelers, and others with legitimate excuses, He has granted a concession to postpone the fast and make it up at a later time. (Qurʾān, al-Baqarah 2:185) Therefore, it is not permissible for a person who has no excuse to give the Ramaḍān fidyah in place of fasting.
Giving fidyah in lieu of fasting is a ruling specific to elderly and incapacitated individuals for whom it is not expected that they will be able to fast in the future.