V 2.91 Shrink Wrap Modifer is Not Allowing Target Selection Total newb. Ubuntu 18.04 Blender 2.91. Uninstalled all previous versions and did a clean fresh install from download.
I've been over this most of the day in these posts and in the documentation, my goal is to simply round the edges of a primitive. The simple method I found via various sources is to create and subdivide the object, then apply a shrinkwrap modifier and set the offset. Here is what happens.
-Create default project cube, subdivide 10 (at first, then tried subdividing multiple times per posts about having enough surfaces.)

Apply shrinkwrap modifer and select the target, but it doesn't allow selection.


(Side note, I do see the Material attached to the first cube, it must be the default. Tried it with and without material attached to the object.)
After trying a few dozen times and digging through more posts and documentation, I found that if I create multiple objects in the collection it allows me to select one of the other objects but the shrinkwrap applies to the FIRST object and incorrectly.
In the screen shot below, &quot;Cube&quot; is the first object and &quot;Cube.001&quot; is the second. I moved Cube aside, created a new cube, and target the second cube with the first's modifier. The result is the modifier is now applied to the first cube (not the target!) and reduces it to a no-dimension object designated by the orange outline select. Note the dimensions pane in this screen shot. You can't see it here, but the offset is set to 0.33mm in the second shot, played around with that a bit.

Remove the modifier the first object re-appears. I also tried, as in many posts, not dividing the cube in advance any applying a Subdivision Surface modifier above the shrinkwrap. What could I be missing? A simple rounding of the cube is all I'm after.
If you just want a cube with rounded corners, the easiest way is to add a bevel modifier, increase the segments to something like 3 or 4 and then add a Subdivision surface modifier set to something like level 2:

If you want the whole cube to be round (like ballooning out), use a Subdivision Surface Modifier, followed by a Cast Modifier set to cuboid:

If you want the edges and corners rounded (like dice), inset every face slightly, and then add a Subdivision Surface Modifier: