Translation:Let me introduce you, this is my best friend.
Okay, but introducing someone to you and introducing you to someone are two different things. I would rather know which one the Chinese actually is rather than quibble about what sounds natural in English.
We're not here to learn English, after all, and not everything translates completely naturally.
Fully agreed, but the point is not that we want to learn English. The point is that we are asked to answer in English every now and then. And it really distracts from learning Chinese IMHO if I answer with a literal translation and get marked wrong (note that in the mobile app, I'm losing hearts for that!) in one question due to the English sentence being unidiomatic ... and then I answer with an idiomatic equivalent in the next question and I get marked wrong because the expected answer is more literal, albeit unidiomatic.
"I will introduce you. This is my best friend." should be accepted.
The first clause ("我給你介紹") is literally "I give you introduction." and would end up being something like "I will give you an introduction." in the future tense in practice in English.
I assume Duolingo uses this translation to keep the statement in the present tense, but they had to change the meaning and use "let" to do that, so allowing answers that use the future tense shouldn't be a problem.
我給你介紹 in Duolingo's defence, is addressing the person to be introduced. People are getting hung up on the tense. Think of this as explaining to your best friend how you introduce them in any given situation, not just in one situation (in the present or future). "This is how I introduce you: this is my best friend."
How about "Let me introduce him to you, he is my best friend." - indeed, the "him" isn't literally there in the Chinese version, but as other comments have shown, the English version doesn't really work by just specifying who someone's getting introduced to without sounding off.
The Chinese expression is very natural. But in English we use quite different grammar to introduce someone. Duolingo often tries to give a somewhat literal version in English, but here it doesn't work. We'll need to learn that this is how to introduce someone, and remember the meeting although the grammar/ structure is quite different