blender does not output solid models by default.
To be fair it's not really a very good tool for designing 3d printable models.
Brilliant for other things, but not great for 3d printing.

So what you probably have is a hollow model with faces.

There are a lot of good solid model design programs around.
openscad and fusion360 being the two most popular for 3d printing.
but even something like freecad and tinkercad, will produce more 3d printer friendly models than blender.