How can I make a white background for fingering number ?
Dorico does this automatically for fingerings that are shown inside the staff, e.g. as you would typically write for guitar left-hand fingering. It won’t do it at all for fingerings outside the staff, e.g. as you would normally write for keyboard, string or brass instruments.
If you go to the Fingering page of Engraving Options and search (type Ctrl+F or Command+F to search within the page) for Erase background you’ll find the option for fingerings within the staff: make sure this is activated.
Have you manually moved these fingerings into the staff? Perhaps you could attach the project itself so we can take a look.
Yes, Here is the filetest.zip (1.8 MB)
If you activate the Position left of notehead property, the fingering will be drawn in the staff with an erased background, and you can then move it in Engrave mode as needed.