Why can't I add a reservation (walk-in) for yesterday?

As this is not an uncommon scenario you are able to add a reservation for the previous day as long as you have sufficient user access rights, you will be able to add a booking for the previous day. (Please check with your manager if you are not sure you have the user rights to do this.) 

Please note that you cannot do this through the Sales View. You will need to go to Reservations > Manage Reservations and then click the Create New button in the top right corner.