Windows stuck in reboot loop due to existing Scheduled Task

You can make a task run as SYSTEM, which means you do not need to provide your own username/password. The only requirement is that you are local administrator.

In order to do this, press the Change User or Group... button and type in SYSTEM, then press ok.

The choice for "run only when user is logged on and Run whether the user is logged on or not" will be greyed out as it is no longer applicable.

Do note, if the task that is running requires your username/password to run (for example because it resides on a network share) SYSTEM will not have access to that and even though the task will run, it will fail for that very reason.

Have you already tried navigating to C:\Windows\System32\Tasks to see if it appears there? If it isn't visible, try performing a search within that folder hierarchy. Just to double-check, is it possible that the task name you chose matches a default/built-in Windows task that already exists?

Also, please ensure that you proceed with caution. Anything you do within that folder hierarchy will have a tangible impact upon your Windows operating system.

Any task within that folder hierarchy should appear in your Task Scheduler Library. If it doesn't and you find your problematic task in C:\Windows\System32\Tasks, you should be able to delete it from that folder and recreate it accordingly.