@Carolyn745 Assuming you checked your availability settings to make sure that it isn't marked as only available for 3 or 6 months into the future, l am able to click on the first day of a period I want to block or open, drag through to the last day, which then highlights that whole period, and then make them available or blocked on the right hand side of the page.
This is on the desktop version, not app, using Chrome browser, on the monthly calendar view.