Adobe Animate fill tool not working

This is an older tutorial but I think it could help. 

https://www.youtube.com/watch?v=DyxG1CZ7HEE&t=68s

Make sure your artwork (assuming you created it using Animate vector tools) does not have any gaps. Also, you may have used Object Drawing mode when the strokes or shapes were drawn. If so, even if they are overlapping, they are separate objects and therefore not a contained shape that could be filled. You can select them all and break them apart or go to Modify > Combine Objects > Union and then use the Fill Bucket.

Also worth nothing the Fill Bucket has sub-selection setings to close small, medium, large gaps which could also be your issue.

open a new fla and start a new as3 project

save to a desktop directory

select the rectangle tool

draw a rectangle on stage

select the paint bucket tool and select a fill color

click the on-stage rectangle

any problem?