The error codes are printed at the bottom of the sheet that lists the orders inputted, and they are listed as follows --x y z--, where x, y and z are:
x - The section the error occurred in
y - The order number where the error occurred
z - The error itself (see section 10 in the rulebook)
So, for example, and error 10 1 6 would mean:
10 - The error occurred in section 10, Build Ships
1 - The error was in order number 1
6 - The error was 'you cannot build a ship in a barracks' (from section 10.10)