Cura says there were no models

Cura shows a warning headed No Models in File, reading There were no models in yourfile.3mf. The plate stays empty.

Why does Cura find no models?

Cura reads geometry from a single part inside the archive, 3D/3dmodel.model. Bambu Studio, OrcaSlicer and Creality Print give each object its own part under 3D/Objects/ and leave a reference in the root part, the layout the 3MF production extension defines. Cura's 3MF reader does not follow the reference, so it finds an empty model and says so.

How do I open the file in Cura anyway?

Convert it to Cura first. The conversion collects those object parts into the one part Cura reads, and writes Cura's own setting names while it is there. See converting a file.

What if preslice shows no mesh either?

Then the geometry really is absent from this copy: the references are in the file and the parts they name are not. Nothing can rebuild it, so ask for the file again.

Does the file still work in its own slicer?

Yes. Bambu Studio, OrcaSlicer and Creality Print all read the split layout they wrote. Cura is the one that needs everything in the single part.