It mean what it says.
In English or plain speak. I cannot find a device/ hard drive that is boot able or has a boot able OS i can see, or I cannot find the hard drive on the interface selected or the mode it is set to run at.
The causes, in the bios the hard drive you installed windows 10 to is not set as no 1. in your boot priority list.
so check it is set to number 1.
Or the interface mode for your Sata device is not set right. Option of modes should be.
Sata mode
Ahci mode, often the problem is sata mode is selected where its more common for Ahci mode to be selected.
IDE mode.
Raid mode.
Because some bios Sata interface selction mode set to Sata result in the drive in question not booting.
Because the sata driver has to be pre installed in a section of windows setup under add driver.
In any case it will be down to the interface mode, set up in your bios, being the wrong mode selected.
If it`s set to Sata in the bios, Set it to AHCI mode save the settings before exiting the bios and see if windows 10 then boots from the hard drive you installed to it ok.