Well, it can be problematic saying "no" when you're new on a job. It isn't always the best time/place to practice assertiveness.
Do you know whether what they are asking you to do is reasonable and regular practice there? Could you maybe ask to back up the charge nurse for a few nights so you are more comfortable with the procedures and what is expected of you in that role? Maybe talk with your supervisor about how these duties should rotate? (Sorry, I'm not a nurse so I'm taking a bit of a stab in the dark, here.)
You certainly don't want to endanger patients, nor do you want to be a doormat, but at the same time, if at some point you need to step up in responsibilities, you don't want your employer thinking you're unwilling to take on those tasks.
Good luck, this can be tough to navigate.