For Business Central (online/SaaS) Sandbox environments a "DEMO" watermark is always applied to reports.
To run reports in a Sandbox environment you do not need a license. The watermark is added to reports to prevent you from accidentally using the prints from a Sandbox environment and to prevent unfair usage of the services.
For Business Central (and NAV) on-premise environments a watermark is applied to the printed reports if the Document Creator product key is used with a Business Central license that is associated with a different VOICE ID (PSBCID) than the key is licensed to.
This typically happens when a partner's Business Central developer license was uploaded to the Business Central database.
You can resolve this by uploading the correct Business Central license to the database and afterwards restarting the Business Central server instance(s).