If you can’t do it, you can’t do it.
I’ve never been much of a believer in the notion of “fake it ’til you make it.” While I’ll concede that it is occasionally possible to bluff your way through a workday (or even an entire career, depending on the field), that’s not really a safe or healthy way to conduct yourself. I get that we all want to look productive and useful within our places of business, but if you can’t do a particular kind of assignment, you’re setting yourself up for a 50/50 shot of crashing and burning.
There’s an old workplace idea that you should never, ever say no when offered some kind of assignment or work opportunity. If you turn down work, especially as a newbie, that means you’re not a “team player” or whatever the outdated corporate jargon is. No one can do absolutely everything; if the assignment is only tangentially related to your skillset, chances are good that the assignment is not going to get done promptly, and it probably won’t turn out very good either. If you were hoping to appear reliable by accepting work outside of your wheelhouse, an outcome like that is going to do the exact opposite. Ergo, if you’re requested to do something that you’re reasonably certain you can’t make happen, you have every right to decline.
Even if you turn down the assignment, you can still score some brownie points if you’re careful about it. Give a proper explanation as to why you can’t do the assignment, whether it’s due to a lack of time or relevant skills. Suggest other coworkers with more relevant skillsets that may be able to handle it properly, and add that if any part of the assignment is relevant to your skillset, you’re more than willing to assist. Many hands make fast work, after all, but again, that’s contingent on you having the time and energy to have that on your plate. If you’re overburdened, make it clear.