Cura says there were no models

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

Why does Cura find no models?

Bambu Studio, OrcaSlicer and Creality Print give each object its own part under 3D/Objects/ and leave a reference in the root part, 3D/3dmodel.model, the layout the 3MF production extension defines. Cura 5.10 and earlier read only the root part (ThreeMFReader.py), so they find an empty model and say so. Cura 5.11.0 and later follow the reference (ThreeMFReader.py), so updating Cura is the first fix.

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.

What does Failed to load mean?

Failed to load yourfile.3mf. The file could be corrupt, inaccessible or it did not contain any models. under Unable to Open File (ReadFileJob.py) means the reader stopped with an error, not that it found an empty model. The shorter The file could be corrupt or inaccessible. fires before reading starts. Open the file in preslice: if it opens, convert it to Cura as above; if preslice names a missing part, the file is damaged.

What does Model Errors mean?

The highlighted areas indicate either missing or extraneous surfaces. Fix your model and open it again into Cura. (SolidView.py). The file opened and a mesh has holes or extra faces. See repair a .3mf.

What does contains an unknown machine type mean?

Project file yourfile.3mf contains an unknown machine type, then Cannot import the machine. Models will be imported instead. (ThreeMFWorkspaceReader.py). The Cura project names a printer your Cura does not have, often a plugin printer or one from a newer Cura. The models load onto your current printer and the project's settings do not. Install that printer, or update Cura, and open the project again.

Does the file still work in its own slicer?

Yes. Bambu Studio, OrcaSlicer and Creality Print all read the split layout they wrote. Only an older Cura needs everything in the single part.