Solution found.
I have missed the initramfs part.
When we change from a clear to encrypt environment, we must configure the initramfs.
a)After booting from a usbkey with iso image(slackware iso is ok, debian is ok too)
b)Open the encrypted md
cryptsetup luksOpen /dev/md0 luksmd0
c)Mount the eventual lvm
vgchange -ay
mount /dev/yourvgname/rootlv /mnt
d)mount those dirs as bind from "fakeroot"
for i in dev proc sys
do mount -o bind /$i/ /mnt/$i
done
e)usually boot is separate partition
chroot /mnt /bin/bash
mount /boot
f)Important..we edit the /etc/crypttab,the uuid must be the uuid of encrypted dev in my case is /dev/md0, the uuid is taken from blkid NOT mdadm, another way is to use cryptsetup luksUUID /dev/md0
luksmd0 UUID=21d33599-a220-zddg-1b1f-b2ae82fc0856 none luks
g)We have to edit those two files,in this one we insert the kernel modules(ext4,md,raid10,raid0,etc..)
/etc/initramfs-tools/modules
in this other we simply enable cryptsetup
/etc/cryptsetup-initramfs/conf-hook
h)now edit /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX="root=/dev/mapper/debian2--vg-root rd.luks.uuid=21d33599-a220-zddg-1b1f-b2ae82fc0856 rd.luks.name=21d33599-a220-zddg-1b1f-b2ae82fc0856=luksmd0 rd.luks.crypttab=no rd.lvm.lv=debian2-vg/root rd.luks=1 rd.md=1"
i)if needed edit fstab and make changes
vim /etc/fstab
m)now mkinit
update-initramfs -cuv -k all
n)and finally grub
grub-install --recheck /dev/sda
grub-mkconfig -o /boot/grub/grub.cfg
if use efi
mount /boot/efi
grub-install --target=x86_64-efi --efi-directory=/boot/efi --recheck
grub-mkconfig -o /boot/grub/grub.cfg
If all is ok and no error given
umount -a
exit
reboot
