This means that the computer cannot find a valid system folder.
You could try to reset the PRAM in order to see whether that makes a difference:
If resetting the PRAM does not change anything, you would have to start the PowerBook Duo from another disk (such as a Disk Tools floppy). This way one can check the contents (if any) of the internal hard disk. If necessary, one can try to repair the hard disk structure and update the hard disk driver software. It would also be possible to reformat the hard disk and install a new operating system.
Can the sound of a spinning hard disk be heard?
Since this model does not have a built-in floppy drive, do you have access to the external drive (or possibly a DuoDock)?
This PowerBook Duo 280/280c can operate with the following Mac OSs.
Original OS System 7.1 (PowerBook Duo Enabler v2.0) Maximum OS Mac OS 8.1
If you need Mac OS floppies here are some sources.