If you use a tool like thomthom's 'Solid-Inspector' it will show the issues.Capture.PNG
Other tools like my 'SolidSolver' can try to fix issues... BUT these tools can often fail because the errors are often too convoluted... a manual fix is then needed...
Your form has several such difficulties...
A manifold 'solid' is best be thought of as an object that contains only faces and their edges, AND those edges ALL have two faces - no more, no fewer.
So that excludes all faceless edges, single-sided faces that will end up somewhere as a 'flap' with an edge with only one face, punched 'holes' in faces that will inevitably have an edge with only one face, internal 'partition' faces within twin-skinned 'walls' that will have some edges with three or more faces etc...
Your form has several problems like those listed above...
You can see them using Xray mode, and Section-Planes to cut the object etc during using 'Solid-Inspector'.
Then you can select+delete to get rid of unneeded edges/faces etc.
There are also a couple of tiny very narrow internal flap-faces, because not all of your faces are exactly 'axial' - use a Style that shows edges 'by axis color' to see these better, zoom in and erase them...
If you make it a group and select it then 'Entity Info' says 'Group' BUT IF it's manifold it says 'Solid Group'...
Here it is fixed Capture1.PNG