"Do you have a taxi?"
있다 means to be / to exist, and 느냐 is an old-fashioned way of asking a question. I would suggest using the honorific ending for questions ㅂ/습니까 (있습니까?), or the informal one 니 (있니?), or even the regular conjugation 있어/있어요 if you want to sound more natural.
I thought the 느냐 form was for reported questions i.e. someone asked if u had a taxi