Is the below sentence correct?
I will confirm once I’m at the office.
I’m trying to tell someone that I will confirm later because I’m not at the office meanwhile. I could not digest “once” followed by “I’m”. That is why I was confused.
In this context, once is acting as a conjunction, which is used to connect two clauses to make a sentence. The two clauses are
I will confirm
I’m at the office.
The two clauses are grammatically correct, and once adds the meaing that the first clause will take place as soon as the second clause occurs.
According to the Cambridge Dictionary, once can be an adverb or a conjunction. The adverbal usage is more common and has a different meaning: maybe that's what is making you uncomfortable about the sentence. You could replace once with the conjunction when, and the meaning would be almost the same, but without the sense of urgency.
I will confirm when I’m at the office.