If your default table turn time of bumps up against your close time then you will see "Make manager booking" against the later time slots. (Only a manager can give a customer a table turn of less than the table turn time you have set. This is deliberate in simpleERB so that customers are not given 'false promises' by staff.)
You can fix this by shortening your default table turn time, extending your opening hours so that your 'close' time allows for enough time for the last booking but you may prefer to do a more nuanced adjustment at:
Advanced table turn admin
Here you can shorten your table turn time for later in the evening.
e.g. if your close time is 11.00pm and you are making a booking at 9.45pm then you need a close time of less than 75 minutes. You might normally allocate 90 or 120 minutes per booking.