Is it a mono alphabetic substitution code?
I think that should be obvious from the instructions. And HAL’s cryptogram helper only works with monoalphabetic substitutions ciphers, with the restriction that no letter can substitute for itself. I have an octave/matlab script that converts plaintext to MASC, with that precise restriction, if you think you would like it.
These cryptograms are meant for amusement, and provide a convenient gathering place. They are not intended as any deep intellectual challenge.