Okay thank you for the tutorial link. It was going good but this is my new error (following their steps):

ubuntu@ubuntu:~$ sudo chroot /mnt
root@ubuntu:/# grub-install /dev/sda
/usr/sbin/grub-setup: warn: This GPT partition label has no BIOS Boot Partition; embedding won’t be possible!.
/usr/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
/usr/sbin/grub-setup: error: will not proceed with blocklists.
root@ubuntu:/# grub-install –recheck /dev/sda
/usr/sbin/grub-setup: warn: This GPT partition label has no BIOS Boot Partition; embedding won’t be possible!.
/usr/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
/usr/sbin/grub-setup: error: will not proceed with blocklists.