hi how do I remove this white space?
https://imgur.com/a/5MvLnYe

It may be that your lineart has anti-aliased edges, i.e. not sharp, so the Fill Tool hits the semi-opaque edge and stops.
You can deal with this by using the Grow Selection option in the Tool Options docker.
At a guess, for your image, I'd say 4 or 5 pixels should make it reach just under the lineart. You have thick lineart so 6 or 7 may be ok.
Try it and if it's not enough or too much, simply Undo and try again with a different value.