The selected date is in the past

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

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

In example, if you are based in Australia (UTC +10) and the timezone of your server/website is set to America/New York (UTC -5), the system will use a time 15 hours in the past compared to your area.
So, if you click the current day at 9:00 AM, the server will consider it as it was 6:00 PM of the previous day.

In order to fix this issue, you should try to adjust the timezone setting from the Settings > General page of your WordPress administrator panel.

When you click the current day, you might not see some times that instead should be there. This happens in case those times are in the past compared to the website timezone.
If some times are not displayed, even if they should be actually available, this could be the same timezone issue listed above.