I found what's the issue.

For just watching the flux density in specific line in air gap area(2D).

I draw a line then followed the guidance someone posted

1: quantity / B

2: scal? / scalarX/Y/Z

3: geometry / polyline1

4: value

By the end, it works ignoring the 3rd&4th step, it can be saved as any name.

I just wonder it should work logically.