Every place in the program that you want a zero you must enter a -1. 0 is nothing and does nothing.

In this case - In the details for the item that is getting the special depreciation, you must scroll down to Special depreciation allowance: 1=yes, 2=no [Override] and enter a 2 for no. 

OR

Scroll down to Current special depreciation allowance (-1=none) [Override] and enter a -1 for  zero.