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
By the end, it works ignoring the 3rd&4th step, it can be saved as any name.
I just wonder it should work logically.