我决定重新安装 Ubuntu 系统。Ubuntu 已正确安装（我猜），但 grub 安装失败。我尝试运行启动修复，它说我应该粘贴以下命令：
sudo chroot "/mnt/boot-sav/sda6" dpkg --configure -a
sudo chroot "/mnt/boot-sav/sda6" apt-get install -fy
sudo chroot "/mnt/boot-sav/sda6" apt-get purge -y --force-yes grub*-common grub-common:i386 shim-signed linux-signed*
不幸的是，第一个命令的输出是
Setting up shim-signed (1.21.3+0.9+1465500757.14a5905.is.0.8-0ubuntu3) ...
Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.
dpkg: error processing package shim-signed (--configure):
subprocess installed post-installation script returned error exit status 1
Setting up grub-efi-amd64-signed (1.74+2.02~beta2-36ubuntu11) ...
Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.
dpkg: error processing package grub-efi-amd64-signed (--configure):
subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
shim-signed
grub-efi-amd64-signed
据我所知，问题出在 UEFI，但我不知道如何解决这个问题以便 grub 再次运行。