make sure to do a few things. First rename the bios to something easy like "new.rom". Then make sure to type the correct command. There are spaces that people often forget, as well as the "i". I'll type it out to demonstrate. The _ will represent spaces.
afudos_/inew.rom_/pbnc_/n
Also make sure you are using the engineering version of AFUDOS (found in the P5E thread below in my sig).