Anyone know why this error "Gateway Rejected: avs" is happening when I go to pay with my debit card?

You should probably call Rover 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] to get this resolved.

For what it's worth, AVS means address verification system. If rover (or whoever is processing the transaction) caught that error they would ask you to check the address you entered, or call your bank to report a mismatch. It could be a system problem too, such as misconfiguration of the interface with AVS.