First question, just to be sure: did you install LibreELEC in EFI or legacy mode?
Using another PC does not work, you have to modify the EFI variables on the Lenovo.
Start the LibreELEC installer again and type live at the syslinux boot prompt. Configure the network in the graphical interface, Install SystemTools addon from the LibreELEC Repo. Start System Tools addon to get a shell. Use efibootmgr omitting the sudo command.