Unable to install GRUB

I got the same error message. The installation configuration that led to this message is as follows:

/dev/sda1        ntfs :windows partition
/dev/sda2        ntfs :windows partition
/dev/sda3        ntfs :windows partition

/dev/sda5             :swap  partition 
/dev/sda6        ext4 :/     partition
/dev/sda7        ext4 :/home partition
/dev/sda8        ext2 :/boot partition

So, what worked -after lots of tests- is the configuration where I have changed the ext2 format of the /boot partition (/dev/sda8) to the ext4 format. I don't know if the problem was really in that point but at last it worked. I hope this will be helpful.

I don't believe grub reliably supports booting a btrfs partition, you'll either have to use a separate ext4 formatted /boot or just use a ext4 /.

I think trying to use grub with a btrfs / works in some cases but not others. It's not very well tested yet.