How do I tell the owners I want to cancel the next booking they have with me because the dog isn't a good fit?

The owners asked me last minute to take their dog for the weekend as a trial weekend and have booked me for 9 days the week after. They wanted to meet and greet and from there give me the dog to take home. On the meet and greet there were no problems. But this weekend he constantly wants to play and bullies my dog to try and get him to play even when my dog is sleeping or growls. When I step in for my dog he barks at me or mouths my hand or clothes to play. He steals things around the house. He jumps on all dogs and people and the only time he settles is after extensive walking. He can't sleep alone so I sleep with him but even then he doesn't settle. I understand he's 1.5 years old but this dog has never been taught boundaries or been told no which makes his behaviour difficult to manage. im hoping the owners don't think we expect too much but it's unfair on my dog and family.