Can't get rid of this 'BOOTMGR is missing' error.I know this is a common problem and I tried googling it.And most of the solutions include plugging a boot device to reboot.Tried that
Cloned Drive BOOTMGR Missing Windows 10 (Solved)
Cloned Drive BOOTMGR Missing Windows 10 (Solved)
Cannot boot from fresh install SSD - Microsoft Community