Between the link that @Silversitters gave and the below, it doesn’t look like you’ll be able to get your money back I’m afraid. There’s a request a refund button on the below link, try it to check.
https://support.trustedhousesitters.com/hc/en-gb/articles/360001741237-Can-I-get-a-refund
Your membership is here to be enjoyed and so if you haven’t used it within 14 days of joining or auto-renewing, you can cancel it. This means that you can request a full refund as long as you have not either applied for a sit or had live dates published on your home listing within these 14 days.