"I am not going to the party because I am dirty."
Translation:Nie idę na imprezę, bo jestem brudny.
The word "idę" has nothing to do with walking or any other way of transport in this context. It indicates, the same way as the English phrase "I am going" does, the intention of attending the party (or not).
The translation "Nie jadę na imprezę, bo jestem brudny" provides more information than the original sentence, therefore it should not be accepted as a correct one.