Fill tool not filling

I tried filling the color on my drawing frame but somehow it wont let me, I check for small gaps but there isn’t any gaps. But I’m thinking I draw too many lines? So I tried drawing a small sketch just only in one line but somehow didn’t fill also, but in shapes it does. I’m confused, does anybody experienced that, if know how to fix it please explain

The fill tool (and most wick logic) doesn’t see lines but instead areas. You can use the brush tool to mark limits.