Hi Brian,
We need some more information.
It should be working for you... we just have to find out why it is not.
The first thing I would do would be to make sure the shape you are working with is closed... so that fill colors will work.
If you aren't using the paint bucket/fill tool and can't paint with a brush... perhaps you are actually painting but using the same color as the background so it doesn't look like you are drawing/painting.
To remedy that, make sure you select a Style that has a different color.
Too many variables at this point so please let us know more about what you are experiencing.
If you can capture your video (using a program like OBS Studio... screencast o matic... etc.) that will tell the tale.