That is probably because metro can mean a subway and a meter in Spanish.
Edit: To explain further because of the downvote I received I think that Duolingo is adding metro in and not checking which meaning of metro it is referring to.
Exactly - this happens a lot. It can get really confusing when one verb conjugation crosses paths with the conjugation of a completely different verb.
I'm curious if this would also be the term used for the "L" in Chicago or light rail in a place like San Diego. Is it any kind of dedicated rail line for commuters, or specifically an underground system? Native speakers, what say you?