This happens to everyone, forever
Not just in your first month. Bartenders with fifteen years behind them get asked for regional drinks, family drinks, and things somebody had at a resort in 2011. The skill is not knowing every drink. The skill is handling not knowing one without losing the room.
The move
Ask. Directly, once, without apology.
"I don't make that one — how do you like it?"
That is the whole script. It takes two seconds and it works because of something new bartenders consistently get wrong: guests are not annoyed by this. People who order obscure drinks are usually delighted to explain them. What annoys people is a drink that arrives wrong, and worse, a bartender who clearly guessed and then acts as though they didn't.
The version that fails is the apologetic one — "sorry, I'm really new, I don't know that, sorry." Now the guest is managing your feelings. Ask like a professional asking a clarifying question, because that is what it is.