Method 1: Disable test mode with "set testsigning off" command line

Step 1: Click Search icon on taskbar and input "cmd" in search box. Choose Command Prompt from search results, and then select Run as administrator on the right, entering CMD panel.
Step 2: Type "bcdedit /set testsigning off/" command and then hit Enter key.
Step 3: This operation has been finished if you get the following message.
Step 4: Click Restart button to reboot your computer and you have exited the test mode when you enter system again.

Method 2: Disable test mode with "set nointegritychecks off" command line

Step 1: As in Step 1 of Solution 1, run Command Prompt as administrator.
Step 2: Input "bcdedit /set nointegritychecks off", and then press Enter key.
Step 3: Wait for a while and you will see the information "This operation is complete successfully".
Step 4: Reboot your computer to disable Windows 11 system test mode completely, and watermark disappears from screen.