STL facet bodies cannot be meshed.
Open the STL file with SpaceClaim where you can create geometry that can be meshed.
The method that requires no effort also has the worst outcome. Right click on the facet body and select Convert to Solid. This creates a solid where every facet is converted to a face and is undesirable for obtaining a quality mesh because there are too many faces and the elements are likely to be of poor quality. Don't do that.
All other methods require a large effort, but have the possibility of creating a high quality elements.
This discussion shows a method to convert an STL to geometry.
Since you have a repeating pattern, I suggest you use three pairs of parallel planes to cut a unit cell out of the array. After you have converted a unit cell, you can use the pattern tool in SpaceClaim to rebuild the array.
Another approach is to construct the geometry from scratch in SpaceClaim. Put 8 spheres on the corners of the unit cell, connect them with 12 cylinders then blend the edges between them. Use three pairs of parallel planes to trim back to the unit cell (1/8 of a sphere/cylinder) then use the shell command to hollow out the inside. This is what I would recommend. The STL can coexist as a visual reference to adjust the size each feature as you build it.