I would be suspicious of a breeder who doesn't allow you to visit their kennel. However, there are exceptions at times. Some breeders are worried about their delicate puppies catching diseases from visitors. Yes, it helps to leave shoes outside, wear surgical booties and cleaning with disinfectants post visit, but some breeders may be reluctant to show their puppies when very young (like under six-weeks-old) to prevent stress to the mom and for disease control. I would inquire whether there are good reasons why you cannot visit, and whether it's a temporary thing.