Earlier today i bought a optiplex 990 from a local reuse center (has windows 10 pro installed) but everytime i try and boot it up i get

PXE-E61: Media test failure, check cable

PXE-M0F: Exiting intel boot agent 

No boot device found. Press any key to reboot the machine

The cable is secured and in the right place . i switched from legacy to UEFI. I ran a system diagnostics and it said no problems found. What's wrong and what do i do ?