What should I do, as I vowed to fast specific days but was unable to fast them?

Vowing (nadhr (definition)) is not encouraged, as the Prophet, may Allah bless him and grant him peace, forbade it. If the vow is to fast and the desired outcome is achieved, then it is obligatory to fulfill it, based on the saying of the Messenger of Allah, may Allah bless him and grant him peace: "Whoever vows to obey Allah, let him obey Him." If the one who made the vow is unable to fulfill it due to illness or similar reasons, then he must offer expiation for breaking an oath (kaffarat yamin (definition)). However, if it was merely an intention to fast without verbally expressing a vow, then nothing is obligatory, because a vow is not legally binding except through verbal declaration.