You may receive an error message when trying to schedule a patient into an open block that states:

Operatory is already scheduled for requested time!

Note which operatory (the columns in the Appointment Book) is affected.

Navigate to File > Maintenance:

Select the double arrow button >> to open the operatory selection menu:

Select the operatory that is generating the error:

Verify the date, then select Start to fix the operator:

You will see a confirmation message and then you should be able to schedule the block: