I forgot to press the button to start my walk on the app. Will I have an issue getting paid if there is not a record of the walk on the map tracker? If this is an issue, how do I fix it?

Starting and completing a Rover card is only mandatory for recurring bookings, a one-off booking you will be paid without issue. If it's recurring I'd call Customer Support [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 they can update the booking to show you completed the walk.