A ‘nested’ instance is a component inside another component [or group].
A group can also be ‘nested’.
A ‘solid’ object must consist of only faces and edges, and every edge must have two faces - no more, no fewer.
If your ‘container’ has nested content, edit it [double-click it or use the menu item].
Use the Outliner window to show the culprits.
Select them and use the Explode function in the context-menu.
Once the container has only edges/faces in it will either be a solid, or at least you can edit it to achieve this.
With nested objects it’ll never be a solid.
Here’s the fixed ‘solid’ version.
All nested parts have been exploded.
Also Un-smoothed it, and finally used a Plugin to Delete-Coplanar-Edges…
The second SKP is very similar…