To set Staff ID Number automatically go to System Settings > General Setting >Staff ID Auto Generation and set Auto Staff ID from Disabled to Enable.
To set values for Staff ID No. Auto Generation here it is recommend to set prefix any alphabet character like SI and if you select Staff ID No Digit as 4 so you must set ID Start From value of 4 digit like 1000 or 1001 otherwise it will show error. Also make sure auto generated Staff ID no. will not conflict in future with any existing Staff ID no. (with manually entered Staff ID No. value).