Hi Juan, there are a few different things going on here.

First of all, there is a curve object right in that area, it will select first if you move over there and click. This will interfere with selecting parts of your solid or surface objects, so select that curve object and delete or hide it to get it out of the way so it does not interfere with your selection.

Next, the bottom and side pieces are different objects, you would normally want to join these together to make one solid object before chamfering it. However, in this case there is a problem with the join - some of the edges are close enough to join, but one pair of edges are 0.012 units apart which is not close enough to join, so after joining it does not form full shared edges in all places, that will interfere with chamfering.

But one way to make progress without joining is to use surface/surface chamfer instead of using edge-based chamfer.

To use surface/surface chamfer, select all those pieces and use Edit/Separate to break them apart into individual surfaces. Now you can select 2 surfaces at a time and run Chamfer on it, and this will succeed at creating chamfer surfaces. After that you will have to do some surface trimming to make progress.

I have attached here the results of doing some of this manual trimming, but you can see that there is some difficulty in the spots where the different chamfers come together - it can be difficult for MoI to automatically fill in corners like these. In this case you'll have to decide how you would like these corner areas to look and trim and create surfaces there to fill them in.

Actually it looks like the part that creates the most difficulty for chamfering is the place along the sides where 2 surfaces touch each other, they are not quite smoothly connected to one another, they meet at a creased angle but the angle is very shallow. This kind of situation creates difficulties for operations that calculate the intersection between surfaces, which fillet and chamfer do.

If you could construct that as one larger piece so that it is smoother it would probably be easier to chamfer it.

- Michael