3D puzzle, magic knot, magic cube

You need 6 parts for a knot. I printed all 6 together with no support geometry.

For assembling: If you add one building block after the other, then it is impossible to add the last building block. Instead, you have to form 2 mirror-inverted halves of 3 building blocks each and push these two halves together over the central axis.

With a small cube, a skilled person can do it alone.
But if you print the building blocks in the maximum size that your printer allows, then it is hardly possible for one person to assemble them alone. The large version can only be assembled by a team and this variant is therefore suitable for a team building exercise. In extreme cases, 6 team members each receive a building block and they have to assemble it together. The shared experience of success strengthens team cohesion.