You need to generate an openssl secret with:

openssl rand -hex 32

and include the following lines into your /var/www/peertube/config/production.yaml file after the webserver section:

# Secrets you need to generate the first time you run PeerTube
secrets:
  # Generate one using `openssl rand -hex 32`
  peertube: '[put output of openssl rand -hex 3 here]'