Windows 10 no bootable devices found

Hi!

I have a Dell G5 5587. I am selling this Laptop on so my friend recommended use a software called Active KillDisk to erase the laptop. The pic attached is before I reset the laptop and the error came but after I reset it via killdisk.

When I reset my laptop now it comes up with the error no bootable devices found. Critical Error Message

I just want to simply reset/remove everything from this laptop as new. 

Do I need to re-install Windows 10 or similar? When I ran the system tests on pre-boot system performance check, no errors were found. It said you can do an advanced scan that would take hours but I said no

Thank you!!