Your model has quite a few problems that would prevent it from being printable.
A lot of the geometry in the model is loose. It should all be inside the group. There are also a number of reversed faces that need to be corrected. You should have no blue back faces exposed.
You can select all of the loose geometry and cut it to the clipboard. Then open the group for editing, right click and choose Paste in place. Then you need to select all of the geometry, right click and choose Intersect Faces>With Model. After that, hide the bottom face and get rid of the internal faces and edges. Some are shown selected below.
This spot is likely to be a problem. Either there needs to be a gap or you need to pull the wall back over the end of the rail.
There’s also internal faces in places like the end of that rail, shown selected, below.
another bad spot is in the 3D text. The font you chose connects the leg of the K with the foot of the A at a single edge. Either separate them or make them overlap. I just drew a straight line segment down at the bottom of the K and use Push/Pull to push away to create a gap.
Since this is a school project I can’t do the work for you but this should help you get it sorted so you can print it.
Once you get it sorted, you should get a pretty nice print. i.materialise seems to like it, anyway.