If the option to download your video is not available or you be presented with the message, this will mean you on a Synthesia Free Plan and therefore do not have this feature available.
Should you wish to download your video:
Click on the Unlock downloads button
Select the Starter or Creator Plan and click Upgrade.
Should you wish to have an Enterprise plan, select the Book demo option.
Notes:
To learn more about the different plans, visit the Synthesia pricing page.