Why does NA appear in the upper left corner of the phone? How to solve it?

That's the weather forecast. Generally, it is a self-contained weather forecast. Because the downloaded weather forecast will prompt you to choose the area when installing and using it. By default, there are no cities. If there is no city, display N/A in the taskbar. Click the weather forecast widget on the main page of the desktop to set the city.

Other mobile phone shutdown notifications also look like this icon. Just light up the notification icon in the taskbar (the one that gently pulls down at the top of the phone).