I'd like to also note, this happens when the Unity Editor or Game Application is actually playing, (Check using Application.IsPlaying).

However, upon using [ExecuteInEditMode] to see what the Run-Time NavMesh Building is doing, it actually works without the error of: 'Source mesh Combined Mesh (root: scene) does not allow read access.'

So again, if anyone can replicate this scenario to confirm that would be a start.

Hi, I was doing some tests, and it turns out that having the objects marked as 'Static' makes the operation fail, as having the 'Static' objects will produce this error message: RuntimeNavMeshBuilder: Source mesh Combined Mesh (root: scene) does not allow read access. This will work in playmode in the editor but not in player
UnityEngine.AI.NavMeshSurface:BuildNavMesh()

Only upon making the objects Non-Static, does it resolve itself. - However Static objects are very handy in taking advantage of the 'Batch-Rendering' that Unity offers, which others may find useful.

In conclusion, currently as I see it or in my personal tests, to utilize Run-Time NavMesh baking, objects Must at this moment be marked as 'NON-Static'.

Is this something that you can replicate as well to confirm?