The realtor or landlord can’t do anything if your door is locked and they can’t get in. And they’re not going to file an expensive court case because they had to come back Sunday or Monday instead of Saturday.
You have the right to privacy. You could have been having private issues at a particular time and needed to reschedule a visit for another time. The landlord doesn’t have the right to come anytime he wishes. He needs to coordinate with you and obtain consent for the time.