OrcaSlicer says 3MF not supported

OrcaSlicer opens the file and tells you The 3MF is not supported by OrcaSlicer, loading geometry data only. Your parts appear on the plate and the settings do not.

Why is the file not supported?

It was written by PrusaSlicer. OrcaSlicer knows the format well enough to take the mesh out of it, but a PrusaSlicer project keeps its settings in a different file under different names, so it leaves them.

What exactly did I lose?

Layer height, temperatures, speeds, wall and infill settings, supports and the filament choice. The geometry, its size and its placement are all intact.

How do I keep the settings?

Convert the file to OrcaSlicer before you open it. Each PrusaSlicer key is written under Orca's own name, perimeters as wall_loops and fill_density as sparse_infill_density (Orca's own PrintConfig.cpp defines both), and the result opens as an OrcaSlicer project. See how converting works.

Can I set it up again by hand instead?

You can, and then every value is an OrcaSlicer default rather than the one in your file. Converting keeps what the file had and puts anything it could not keep in a Notes row.

What will still not carry?

PrusaSlicer's layer ranges and its per-part settings. Modifiers, negative volumes, support blockers and paint do carry. See what a conversion cannot carry.