1) This issue can happens in Booking Calendar Business Small or higher versions, if this you have activated using "Check in/out times" (check more about this feature here) at the Booking > Settings General page and incorrectly configured "Check in/out times fields" (usually these fields are empty during this issue.
How to fix.
- You need to be sure that the check in time is older than check out time. For example: Check in: "14:00" and check out: "12:00"
- Or you can simply deactivate "Check in/out times" option, if you do not need to use change-over days.
2) Also this issue possible if you have incorrectly configured time(s) fields at the Booking > Settings > Form page.
You can try to remove any times fields from the Booking > Settings > Form page, or try to reset booking form to one default "Form templates" (you can select one predefined form template at the toolbar, and then click on "Both" button, its will reset your form. After this you can click on "Save changes" button to save these changes.
Otherwise, please use shortcode generator at the right side of the Booking > Settings > Fields page for the fast and correct creation of booking form fields.
3) Other workaround. We do not recommend todo this point, because dropping indexes (as in bellow instruction), will significant impact to speed of loading.
So for fixing this issue, please open the phpMyAdmin or similar tool for the working with DB
and execute this SQL command:
ALTER TABLE bookingdates DROP INDEX booking_id_dates
or this (depend from the prefix at your DB tables):
ALTER TABLE wp_bookingdates DROP INDEX booking_id_dates