When you have trouble with Normals, i would highly recommend the following.
- Go in the Viewport Overlays and activate "Face Orientation"
- You should now see red and blue overlay on your object (blue face direction should always point outwards, red the backside of faces and should only point inwards)
- Go in Edit Mode and press A to select all
- Press Shift+N to recalculate normals outside
- If there are still red areas facing outwards you need to select them manually
- Press Alt+N and Flip those Normals
Now the entire object should be blue and you can deactivate the "Face Orientation"
A little tip, add Flip and show Face Orientation to Quick Favorits per RMB
It's so helpfull to have at hand and saves a lot of time.
Important is that only selected faces will be affected by the normal operations.
Interior faces as the tooltip says are faces were all edges have more then 2 face users!
Means faces that cover holes where more geometry is hidden behind.
Here a little visual explanation about it:
It's obvious that there are faces hidden behind the middle one covering a hole.
I believe you may have generated overlapping faces that lie above/in front of others that are facing outwards.
Try saving the file incremental and then select those interior faces and hide them with H. Should those now hidden faces not affect the mesh, just press Alt+H to show them again and delete them with X, "Faces".