>>213612271 (OP)>on her first day of shootingThis isn't surprising and is quite common.
If the nudity is important, you can't risk the actress changing her mind. And you can't bully/threaten her to take her kit off, regardless of what the contract says, because it's not the 90s anymore. All you can do is fire her for breach of contract, or renegotiate the contract.
If you've already filmed a week of shooting with her, it's financial suicide to fire her given that you'll have to reshoot all of those scenes with someone else, which also gives her bargaining power in contract renegotiations.
Thus, you film it first. She chickens out, you boot her and either delay filming or shoot with her understudy. Insurance might cover it. Even if they don't, it's a lot cheaper than the alternative.