"Basic" is not a brush. Adobe put it in a stupid place. "Basic" means "no brush - basic stroke".
When you attempt to use the Brush Tool and set it to "Basic" what actually happens when you start to paint is the brush changes automatically to a calligraphic brush. Because... "Basic" is not a brush. I'm not meaning to imply you are deficient in any way for not knowing this.. Adobe is for putting "Basic" in the brush panel.
In addition, Illustrator brushes are reliant on the stroke color, not the fill color.
So. why it's not working for you....
Somewhat educated guess...
You opened a raster image in Illustrator. Doing this means the Brushes Panel is empty, other than "Basic". Therefore there's no calligraphic brush for Illustrator to automatically switch to. So, you can't paint. Gotta have an actual brush for the Brush Tool, not "Basic".
Another possibility is you deleted or removed all the brushes other than "Basic" from the Brush Panel.. which would provide the same outcome essentially.