This will fix on a standard installation, if you do manual partition, ask at the forum!
Boot up with our LiveIso.
mount System to /mnt
You need to know which device is the right one check with
sudo fdisk -l
/dev/sdXn should be replaced with the right device information, and depending on your installation you may have or have not a separate /boot partition. If not /boot is a folder under “/” on the same partition as the filesystem root.
mount /dev/sdXn /mnt
this is your / root partition
mount /dev/sdXn /mnt/boot
your /boot partition –> without extra /boot partition you do not need this step!
mount /dev/sdXn /mnt/boot/efi
the EFI-boot (fat) partition
BUT that grub is not in the boot list can be caused also by EFI-Boot-Settings not related to Grub and EndeavourOS / Archlinux then, so re-/installing-/building grub may not solve your problem…
You can also make sure to get second (dual boot) installation rechecked and put into the grub boot menu:
(as root also) If you have dual-boot os-prober may take a long long time to work, the trick here is to mount all other partitions, before using it.
grub-mkconfig -o /boot/grub/grub.cfg
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=