Can't get this model to slice

[curious aardvark]

that's just a mess.
Also would never print without supports in the centre.
oops, turn it upside down and if it were solid it should print without supports _ I think.
It looks like the top part is being treated as solid, rather than a solid with parts removed.

It does slice in simplify3d. But the whole top is solid and the threads do not show in the sliced version.

Blender is definitely NOT recmmended for stl design.
It does work - but easy it is not !

Have a look at tinkercad, fusion360 (free version) or openscad.

And 3d modelling for appearances, is a totally different animal from 3d modelling for 3d printing.
We only deal in solid shapes = most cad packages work in shells and voids.