HappyEasyGo — Refund for flight cancelled by airline

I booked a flight from happy easy go from indore to goa for 2nd Oct 2021. The flight was via mumbai. Air India cancelled indore to mumbai flight and gave me a sealed hand written note for full refund. I applied for refund on happy easy go and now after 3 months they have rejected it saying the note is not applicable. They are not providing any refunds.