How can I ask owners to complete bookings or cancel?

If the client hasn't paid, then you do not have a booking. Do not hold accept a dog or perform a pet-related service without having received Rover's confirmation.

Did you "formally accept" the booking? If so, then you can message these people and tell them that they need to complete the booking process by confirming the payment information. They have a certain amount of time in which to finalize and then it is no longer considered pending, opening your calendar up for that period. If they are having problems in completing it, you can attempt to help or refer them to Rover Customer Support.

You can nicely tell them that you have other requests and cannot hold these dates open and encourage them to complete the process, as it is first-come, first serve.