The error can also happen with geometry that is technically manifold but has self-intersections, duplicate faces, zero-area faces, or bad normals.

Go through these checks in this order.

In Edit Mode:

A → Shift + N

Make sure “Recalculate Outside” is selected if Blender asks.

Then export and try Bambu again.

In Edit Mode:

A → M → By Distance

Look at the message Blender gives you.

If it says something like:

Removed 0 vertices

that's fine.

If it removes hundreds/thousands, you had overlapping vertices.

With everything selected:

F3 → Degenerate Dissolve

This removes faces/edges that have effectively zero area or collapsed geometry.

This is particularly worth trying if you've been moving vertices around manually.

This one is different from non-manifold geometry.

Your object can be manifold and still have two faces occupying exactly the same space.

In Edit Mode:

A → F3 → “Merge by Distance”

again, and then inspect the model in Wireframe (Z → 4).

Also try:

Select → Select All by Trait → Interior Faces

If Blender selects anything, those are faces completely enclosed inside your solid. Delete them if they're not intentional.

Since you're preparing this for Bambu, you can do:

Edit Mode → A → Ctrl + T

Then:

A → Shift + N

This gives the exported STL a very explicit triangular surface representation.

In Blender, select the object and go to:

Object → 3D Print → Check All

If you don't have the 3D Print Toolbox, enable it under:

Edit → Preferences → Get Extensions → 3D Print Toolbox

It can check things such as:

• Non-manifold edges
• Intersections
• Degenerate faces
• Bad geometry
• Distorted geometry

For a model that looks perfectly normal but Bambu refuses to slice, self-intersections are the thing I'd investigate next.

And there's another very good diagnostic: temporarily export the model as STL after triangulating it, then import that STL back into Blender. If the re-imported STL looks different or Blender reports problems, we've found that the topology is the culprit.