Problems with Ubuntu Unity 21.04 (and solutions)
At the end the installation there's problem of installing. grub-efi-amd64-signed (1.158+2.04-1ubuntu37) doesn't get installed, but the system is installed, only you can't boot into it. The matter is, access to the file initrd.img.5.10.0-14-generic blocked for the group, so grub doesn't see it, when creating grub.cfg file -- it doesn't have initrd /boot/initrd.img-5.10.0-14-generic line in it, so the system won't boot, even if you have other distros in the computer and try to boot from their grub.
The roundabout way to get the system running is by clearing the permission problem and adding the line initrd/boot/initrd.img-5.10.0-14-generic to grub.cfg file manually. Once this is done, the system would boot.
This is a problem is usually there in Ubuntu releases, where the initrd file's permission is blocked to the group. So, before before squashing the system to get the squashfs-root file, check if the initrd file has group permissions.
The 2nd problem is it takes quite a long time to shutdown.
The solution to this:
$ sudo systemctl edit cups-browsed.service
and enter
[Service]
TimeoutStopSec=10
to set the timeout down to 10. Then a
$ sudo systemctl daemon-reload
should make it take effect. Now, it shuts down quickly.
(I am copying my post I did in the U-Unity forum. https://www.foss.ubuntuunity.org/forums/discussion/44/problems-with-ubuntu-unity-21-04#latest)
I also noticed that /boot has only vmlinuz-5.8.0-25-generic and initrd.img-5.8.0-25-generic, which are not blocked to the group. But, if one marks updates, the installer makes a problem with the initrd.img-5.10.0-14-generic, by blocking it to be seen. So, it won't be there in the installed system, hence not booting. Either, upgrade the kernel and initramfs to 5.10 and release the iso, or ask the user not to update while installing. You can boot to the kernel 5.8.
In the Ubuntu Unity thread I have a suggestion to other places to upload the iso.