Install windows 10 first. Then install 18.04.5 alongside windows. There is NO  SUPPORT for any Linux unless it shipped from Dell with Linux. There is NO FREE Linux support of any kind from DELL or Canonical. The system must be prepared to take ubuntu BEFORE installing it aka diskmgmt.msc shrinks the windows partition to allow linux to reside on an open area of the disk. Installation type is NEXT  most important step in the Ubuntu installation. Here, select  Something else to install Ubuntu 18.04 along SIDE Windows 10 This is not a single step single reboot water chicken install.  There is support for this it is NOT free.  
Hi, I just bought a  new Dell OptiPlex 7070  and tried to install Ubuntu from a bootable USB and it was shown successfully. But when I  restarted the computer without the USB, I got error "no bootable devices found".I reinstalled once but still got same error even I  tried "Restore Settings"  as "BIOS Defaults"and "Factory Setting" in BIOS.  If I check "UEFI" there is not any  "Boot Sequence" shown in BIOS, see the attached picture. If I click "Add Boot Option", the default file cannot been found. How can add boot option or are there any other options to find the harddriver? Thanks. /Jing   [IMAGE]    
no bootable devices found