To help resolve the issue, perform the following steps. You will either get a message in the lower left corner or a full screen message in gray. The most common reason for this is the installation of some drivers that are still in their testing phase. If you start up your PC and a message or watermark is visible saying Test Mode or Testing Mode, that indicates that the mode has been activated and is running on your PC.
This mode as the name indicates helps test software or drivers that are still being developed or haven't been certified yet.
There is a function of Windows 10 called Test Mode which allows you to install and test drivers that haven't been digitally signed by Microsoft. Are you looking to remove the test mode messaging from the Windows 10 desktop on your PC?