Feeling nervous means you recognise that something is important. Humans feel nervous about all sorts of things, from weddings to wars, because they recognise that what is happening matters. They might be deeply involved or a bystander: if the outcome is important and they’re paying any attention then they might feel nervous.
Your viva is important. You, of course, are deeply involved. The outcome matters. Of course you might feel nervous, and if you do there is nothing wrong.
If you feel uncomfortable as a result then that’s not great, but that’s not the end of the story. You can talk to others to get help and put your feelings in perspective. You can reflect and help yourself to get things sorted out. You can work to build your confidence.
It’s not wrong to be nervous, but it might not be comfortable. If feeling nervous isn’t helping you then consider what else you can do to change how you feel.