You can cancel your reservation within 24 hrs of purchase for a full refund. Your reservation is non-refundable. After 24 hours, purchases are non-refundable. A credit voucher good for future travel will be issued for qualifying changes/cancellations.
Effective May 1, 2021: To accommodate the continued need for flexibility due to COVID-19, Allegiant's change/cancel fee has been temporarily reduced to $25 per person, per flight segment. ... Allegiant does not use third parties or travel agencies to process changes or cancellations to existing itineraries.
If you choose to cancel your reservation, a fee of $75.00 per passenger, per segment will apply and credit will be issued minus cancellation, carrier charges and booking fees. Remaining funds will be applied as non-refundable credit good for future travel on Allegiant Travel.
https://www.helpbuz.com/cancellation-refund-policy-for-allegiant-airlines/