assuming you used gpt partition layout, there is no bootable flag. The concept doens’t exist in UEFI. Instead, UEFI can read boot files on a FAT12/16/32 filesystem.
May I ask which installation procedure you used?
I’d recommend taking a look at this article if you have not already. Especially the part about checking whether you’re booted in EFI mode. Installing EFI grub is problematic if you’re doing it from a legacy environment (it can be done manually, but you may also have to manually add your Linux install to the grub menu as os-prober doesn’t seem to detect EFI installs from legacy boots, and vice versa.
Note that you can either start over with the install, or you can use the grub CD to rescue boot into your installed system, provided that only grub installation failed. (if you can boot the installed root, then you can make the necessary fixes from within the running OS, such as. mounting ESP, installing grub etc).