Answer:
The use of "doesn't just conserve fuel, it also..." is wrong. Wrong grammatical structure.
Step-by-step explanation:
The person should have used "not only... but also..." in this sentence for it to be grammatically correct. The correct sentence is "riding a bicycle to work not only conserves fuel, but also contributes to a person's physical and mental well-being". There is parallelism because in the first part of the sentence a verb is used and in the second part of the same sentence, a verb is also used. This is important whenever you use "not only...but also" in a sentence. We use "not only...but also" when there are two things to mention but the second thing seems to be more interesting or the writer/speaker wants to give emphasis to the second thing. In the sentence at issue, the fact that riding contributes to a person's physical and mental well-being seems to be more important than "conserving fuel". There is more emphasis in the second part of the sentence.