How to send a reminder to a US prof who has accepted my internship offer but didnt reply to my follow-up mail
Two days is no time at all. But if your first request for a letter didn't emphasize the time constraint then you could send a note apologizing for that omission and giving the deadline. 

And if the letter was supposed to be addressed to the institution and not to you personally, it is possible that it has happened or will happen without your knowledge. 

But if you were initially clear about the time requirements then a note after two days is just noise. The prof may be busy, even busy composing the note you need.