Reason : In setting page , “Limit Days of booking” option is set to zero.
Step – 1 : Go to Settings tab of BUP plugin section there you will see the options as shown below
Step – 2 : If it is set to “0” , delete it and save the setting. If it is blank then just save the settings . It will enable all dates on calendar. To know more about this feature refer https://doc.bookingultrapro.com/allowing-booking-for-limited-days-feature