Nerves don’t mean there’s something wrong. They’re just a symptom of you noticing that the viva is a big day.
Your viva is important, so you probably feel nervous as a consequence. Nothing more.
Prepare for your viva, because it is important. Do what you can to find confidence, because while confidence won’t banish nerves, it will help to keep them in check.
Acknowledge that your viva nerves are there. An anticipation of an important event. Nothing more.