Reboot second Linux with efibootmgr

I have LibreELEC is installed on a SSD and an other Linux on a second Disk. Default boot option is LibreELEC. I want to reboot into my second Linux from LE and tried "efibootmgr". There is an option to change the default boot once for the next reboot of the System. When i start "efibootmgr" there is this error:

Quote

EFI variables are not supported on this system.

How can i fix that?

That is not the same. The user is trying to boot with Grub. LE doesn't use Grub and the easiest Way is to use efibootmgr.

LE is booting in the normal way. I though that i can run efibootmgr even LE was not started via efi. That doesn't work? Is there an other way to change the default boot in efi once?