If your website is new, you likely wouldn't have an s_vi cookie and therefore no legacy Analytics Visitor ID. If you are deploying a new web property we fully recommend deploying the Experience Cloud ID Service (formerly Visitor ID Service & Marketing Cloud ID Service).
According to the screenshot above, you likely already have deployed the ID service since you've an AMCV cookie present. Using the code provided in the previous answer will return your Analytics MID value that is used for visitor identification in Analytics for Experience Cloud ID Service users.
_satellite.getVisitorId().getMarketingCloudVisitorID();