There is another way to make tube from curve –registering a bezier circle as bevel object in a curve. But this method needs to tweak scale of the bezier circle.
This is easier because it doesn’t need a bevel object.
As the image shows below, you should set “full” in fill method to make faces around curve.
Then increase “depth” of bevel to make the tube bigger.
Lastly, increase “resolution” of bevel to smooth faces otherwise it remain sharp, low resolution faces.