If you are a Free Plan user:
The Synthesia logo or watermark, will be present on all videos made with your account.
To remove it you need to upgrade your account and re-generate the video.
If you are using the API, a watermark maybe present due to how you have the test parameter set:
When the test parameter is set to true, no minutes are consumed but the video is watermarked.
When the test parameter is set to false, minutes will be consumed and no watermark should be displayed.
Read the test parameter guidelines for more information.