Do you have object mode turned on or something? That would make each line a seperate object and fill would therefore not work. You'd only need one line to accidentally be an object or group and you're not going to get a fill. Try breaking it apart. Another thing that can happen is if you accidentally drew some lines on a different layer. I have done this countless times...
Also, have you set the fill settings (small box at bottom of toolbar when using paint bucket) to 'close large gaps'?
I have found a bug with colour where sometimes the colour is not updating in the toolbar for some reason. When I change the colour in the colour pallet it seems to fix this but sometimes I have to do it a couple of times. Im using a Mac and Animate 2020. Flash has always had its bugs on Mac so I've just learned to live with it! At least it's not crashing every 20 minutes like it used to until about a year ago...