Probably, the host doesn’t have availability for that period. The host’s calendar can be blocked because he may have already confirmed with another traveler for those dates or he may have closed his availability. See the image below:
The dates in grey cannot be selected!
Another reason for this can be minimum and maximum length of stay. Some hosts need travelers to stay for specific periods of time to perform the activities (see the image below). So, make sure your application matches this requirement.