Preparing to read copy for a voice over can be stressful. Knowing that each recording can have an impact on your chances of future gigs, being nervous about the competition, or whatever else that runs through your head can all have a negative impact on your recording. Get familiar with the script, understand what you can, and just be yourself. Trust that your skill and experience that has gotten you this far will continue to carry you.
You can read more tips for preparing for a voice over recording here: https://www.audiolinks.com/blog/how-to-read-copy-as-a-voice-over-actor/