You either know something or you don't.
When you are in a position where you don't know something, then either it is expected of you to know that thing or it is not expected of you to know it.
If you are expected to know something that you don't know, you can either be upfront about it or not. Your choice here really depends on the environment you are in.
No one is expected to know everything all the time. Any basic competency that should be expected of you as a must have is the responsibility of the hiring team at your firm. So the fact that you're there already means you know what you need to know, and if you don't know something then it's okay not to know it but you may eventually have to learn it on the job.
That is, of course, if your firm hires people properly, and if not, then you're essentially a false positive hire, however this is not your fault, but rather the fault of a faulty "successful potential employee competency model" with a non-zero false positive rate.
In which case, this is also not your fault, however at this point your employer needs to determine whether they are better off letting you go and having another go at it (usually in most cases a bad/unproductive decision) or letting you pick up the necessary skill on the job, which in most cases is probably the better option.
On another note, self-confidence is important, and that come what may, believe in yourself and your skills to the point that you can whether whatever comes your way. This doesn't mean that you need to know everything immediately, but that you are confident enough in your learning/adaptive abilities that you can learn whatever it is you will need to be successful in your role.