Redeeming of my booked flights

I booked a flight and paid for it before Covid. I first wanted to cancel it and they said that they would only pay me half of what i paid for my tickets. I them decided to keep the ticket and use it when travel opened again. i have now app**** to use it and they say that there are no flights available for the whole of November 21 or December 21. This is not true as i have gone onto their website and there are so many flights available so i don't know why they don't want to honor their commitment to let me rebeem my flights. TCC ***

Please assist.