It’s not a bad thing if you feel nervous before the viva, or at the start.
It’s probably not comfortable. Nervousness can sometimes grow into feeling overwhelmed or anxious because – but by itself it’s not a terrible state.
Nervousness recognises the importance of something: your viva matters. Success means something.
If you feel nervous, don’t fight it. Focus on the work you’ve done. Remind yourself that your ability and knowledge, your effort and research outcomes are what has brought you to your viva. Focus on all of that and you’ll find enough confidence to put your nerves in perspective.
Yes, your viva is important, so you might feel nervous.
Yes, you did the work, so you can feel confident.