When requesting a house sitter, we are prompted to make at least two requests. And so, it would seem one of the most common functions would be to cancel one of the requests as only one sitter can do the job. I don't see an obvious way to do that.
There is not any way to do this. Rover surely knows this as the topic comes up regularly. My only advice is to call Rover Customer Support at [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team] and ask why there is no such function. I have asked them to cancel a request myself and they've told me that even they can't do it. However, the issue needs to go further than those first-level support. People need to formally request that the website have this function. No guarantees though that Rover would bother since they haven't in all these years.