"Do you need a taxi, Sir?"
Translation:¿Necesita un taxi, señor?
If you are using a pronoun in the question (though it is not necessary) you would write it after the verb. So if you wanted to use 'usted' you would say, ¿necesita usted un taxi, señor? But it is unnecessary and kind of redundant since you are using señor as well, but that would be the correct Spanish grammar - pronoun after verb when asking a question. "Usted necesita un taxi, señor" is taken as a statement, like telling the man he needs a taxi, not asking him.
I reported this because the help text is ambiguous as to why the answer is incorrect.
You used the tú form "necesitas" instead of the él/ella/usted form "necesita".
That seems to be giving two possible problems. I suggest that this should be broken down into two different help strings, something like:
You used second-person tu instead of third person el/ella.
You used informal tu form instead of formal usted.
This would help new learners understand what exactly they did wrong and better correct their thinking.