I have watched vids, read docs; nearly at wits' end.
...help...plz.
https://drive.google.com/open?id=11LKIdsEkWeODjJblDxVRxUVVrmCLpWmR

This is strange because I've duplicated your arrangement and I see grid lines at all levels of zoom. The fact that you have a selction active should not affect the grid or its visibility.

Can you give more information such as your OS, computer specs and graphics card?
What do you see if you zoom in?
Have you made any settings changes since you installed Krita?
Can you retry this with a smaller image, such as 200 x 200 pixels.

All my suggestions are 'long shot' but there may be something that gives one of the developers an idea of what might be happening.