Are you in the line style panel? I’ve done this before not realizing that I am in the wrong fill panel. LOL. Make sure that you are in the panel with the artist palette. If that doesn’t work it could be that the icon you are using has an outline that is considered the fill of the icon. I know that there is a heart from Flaticon that does this and so I have to leave the middle white and let it fill the outline because the outline is considered the fill of that icon. Hopefully that makes sense.