The selected date is in the past

This message is displayed every time you click on a day lower than the current day.

If you are getting this message also for the current day, probably the time zone of your server doesn't match the time zone of your area.

In example, if you are based in New York (UTC -5) and the time zone of your server is set to Australia/Sidney (UTC +10), the system will use a time 15 hours ahead of your area.
So, if you click on the current day at 9:00 AM, the server will consider it as 1:00 AM of the next day.

When you click on the current day, the times in the past will have an orange color, which means they are not available.
If some of your times result in the past even if they are effectively available, it could be the same time zone problem listed above.