If you haven't saved your game process, just exit, and restart the game, then you'd get another chance (Don't say no this time!). Or you have to earn enough money to buy an Adopt Lisence to Marnie (it'd take a lot of time).
If you're using a switch it is impossible, you will not get an option for a new pet. I think maybe if you're on PC you might be able to edit the save file?