To be ‘solid’ a group must contain only geometry - edges and faces.
Every edge must support exactly two faces - no fewer and no more.
The supplied SKP’s main-group contains another nested group, rendering it non-solid [i.e. the open-sided end part].
If you explode that nested-group into the other geometry, then then main-group will become solid.
However, the supplied SKP contains yet another [solid] group [i,e a cube located internally] - which I assume you’d like inside the main-group.
I recommend you select it, then Edit>Cut it, and then edit the main-group and use Edit>Paste_In_Place.
Then explode it, so its geometry now becomes nested geometry inside the main-group.
Exit the edit and the main-group [now containing all the combined parts, as raw geometry], will be solid in Entity Info.
PS: There are three rogue faceless-edges located outside of the main-group, around the perimeter and to the left of the capped-end, but they are easily selected and deleted…
Then the only thing left in the model is the solid main-group.