When I reboot, I get a grub prompt, so what am I doing wrong?:
root@debian:~# lsblk && blkid
NAME                     MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINTS
sr0                       11:0    1  3.7G  0 rom   
vda                      254:0    0   20G  0 disk  
├─vda1                   254:1    0  487M  0 part  /boot
├─vda2                   254:2    0    1K  0 part  
└─vda5                   254:5    0 19.5G  0 part  
  └─vda5_crypt           252:0    0 19.5G  0 crypt 
    ├─debian--vg-root    252:1    0 18.5G  0 lvm   /
    └─debian--vg-swap_1  252:2    0  980M  0 lvm   [SWAP]
vdb                      254:16   0   20G  0 disk  
├─vdb1                   254:17   0  487M  0 part  
├─vdb2                   254:18   0    1K  0 part  
└─vdb5                   254:21   0 19.5G  0 part  
  └─vdb5_crypt           252:3    0 19.5G  0 crypt 
    ├─debian--vg2-swap_2 252:4    0  980M  0 lvm   [SWAP]
    └─debian--vg2-root   252:5    0 18.5G  0 lvm   
/dev/mapper/debian--vg-root: UUID="787e196d-c852-491e-9583-34d1bc621ace" UUID_SUB="5246486c-8c05-4d72-b494-b9bbf2e62825" BLOCK_SIZE="4096" TYPE="btrfs"
/dev/vdb5: UUID="584aa6ad-c8c0-43e5-b32d-eda94ea8501b" TYPE="crypto_LUKS" PARTUUID="a36f14d2-05"
/dev/vdb1: UUID="55762e61-a486-47b9-92cc-29d597b37312" BLOCK_SIZE="1024" TYPE="ext2" PARTUUID="a36f14d2-01"
/dev/mapper/debian--vg2-swap_2: UUID="662d79c0-2db1-4247-bb77-7d05dfb6917c" TYPE="swap"
/dev/sr0: BLOCK_SIZE="2048" UUID="2023-10-07-11-48-54-00" LABEL="Debian 12.2.0 amd64 1" TYPE="iso9660" PTUUID="515b3f36" PTTYPE="dos"
/dev/mapper/debian--vg-swap_1: UUID="3f8a7e7e-21c1-47d3-a19a-72dfe9f8b164" TYPE="swap"
/dev/mapper/vda5_crypt: UUID="UmwnNd-voGV-v1lc-eKPP-nLeP-cwpX-tXaA7V" TYPE="LVM2_member"
/dev/mapper/debian--vg2-root: UUID="787e196d-c852-491e-9583-34d1bc621ace" UUID_SUB="2c6c20fe-abf0-4b5c-9896-3239dbbbe8f5" BLOCK_SIZE="4096" TYPE="btrfs"
/dev/vda5: UUID="c8e57554-de8b-44c0-862a-32cd1e4f92b1" TYPE="crypto_LUKS" PARTUUID="00819e90-05"
/dev/vda1: UUID="c2206369-9b9f-44b3-8319-1d46ed4c559e" BLOCK_SIZE="1024" TYPE="ext2" PARTUUID="00819e90-01"
/dev/mapper/vdb5_crypt: UUID="mtToH2-apOl-L8Wq-UrcM-QmF9-cUBk-7POUGH" TYPE="LVM2_member"
root@debian:~# mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/vda1 /dev/vdb1
mdadm: super1.x cannot open /dev/vda1: Device or resource busy
mdadm: ddf: Cannot use /dev/vda1: Device or resource busy
mdadm: Cannot use /dev/vda1: It is busy
mdadm: cannot open /dev/vda1: Device or resource busy
root@debian:~# umount /boot
root@debian:~# mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/vda1 /dev/vdb1
mdadm: /dev/vda1 appears to contain an ext2fs file system
       size=498688K  mtime=Sun Nov 19 07:55:07 2023
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
mdadm: /dev/vdb1 appears to contain an ext2fs file system
       size=498688K  mtime=Wed Dec 31 19:00:00 1969
mdadm: size set to 497664K
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
root@debian:~# mkfs.ext2 /dev/md0
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done                            
Creating filesystem with 497664 1k blocks and 124440 inodes
Filesystem UUID: 20abf9b7-04fe-48c3-939a-030ebe363b98
Superblock backups stored on blocks: 
	8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409

Allocating group tables: done                            
Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done 

root@debian:~# cat /etc/fstab 
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# systemd generates mount units based on this file, see systemd.mount(5).
# Please run 'systemctl daemon-reload' after making changes here.
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
#OLD_BUT_KEEP_FOR_REFERENCE/dev/mapper/debian--vg-root /               btrfs   defaults,subvol=@rootfs 0       0
UUID=787e196d-c852-491e-9583-34d1bc621ace /               btrfs   defaults,subvol=@rootfs 0       0
#OLD+RM:/dev/mapper/debian--vg2-root /mnt/2nd_root               btrfs   defaults,subvol=@rootfs 0       0
# /boot was on /dev/vda1 during installation
UUID=c2206369-9b9f-44b3-8319-1d46ed4c559e /boot           ext2    defaults        0       2
/dev/mapper/debian--vg-swap_1 none            swap    sw              0       0
/dev/mapper/debian--vg2-swap_2 none            swap    sw              0       0
/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0
root@debian:~# vim /etc/fstab 
root@debian:~# blkid
/dev/mapper/debian--vg-root: UUID="787e196d-c852-491e-9583-34d1bc621ace" UUID_SUB="5246486c-8c05-4d72-b494-b9bbf2e62825" BLOCK_SIZE="4096" TYPE="btrfs"
/dev/vdb5: UUID="584aa6ad-c8c0-43e5-b32d-eda94ea8501b" TYPE="crypto_LUKS" PARTUUID="a36f14d2-05"
/dev/vdb1: UUID="53f0e6f7-7800-c6e1-31db-f89ee06768fa" UUID_SUB="27009b95-ced4-0102-14ec-e5ae5d4719ac" LABEL="debian:0" TYPE="linux_raid_member" PARTUUID="a36f14d2-01"
/dev/mapper/debian--vg2-swap_2: UUID="662d79c0-2db1-4247-bb77-7d05dfb6917c" TYPE="swap"
/dev/sr0: BLOCK_SIZE="2048" UUID="2023-10-07-11-48-54-00" LABEL="Debian 12.2.0 amd64 1" TYPE="iso9660" PTUUID="515b3f36" PTTYPE="dos"
/dev/mapper/debian--vg-swap_1: UUID="3f8a7e7e-21c1-47d3-a19a-72dfe9f8b164" TYPE="swap"
/dev/mapper/vda5_crypt: UUID="UmwnNd-voGV-v1lc-eKPP-nLeP-cwpX-tXaA7V" TYPE="LVM2_member"
/dev/mapper/debian--vg2-root: UUID="787e196d-c852-491e-9583-34d1bc621ace" UUID_SUB="2c6c20fe-abf0-4b5c-9896-3239dbbbe8f5" BLOCK_SIZE="4096" TYPE="btrfs"
/dev/vda5: UUID="c8e57554-de8b-44c0-862a-32cd1e4f92b1" TYPE="crypto_LUKS" PARTUUID="00819e90-05"
/dev/vda1: UUID="53f0e6f7-7800-c6e1-31db-f89ee06768fa" UUID_SUB="acfcbda3-2ec3-da37-2d4b-072e450b33c7" LABEL="debian:0" TYPE="linux_raid_member" PARTUUID="00819e90-01"
/dev/mapper/vdb5_crypt: UUID="mtToH2-apOl-L8Wq-UrcM-QmF9-cUBk-7POUGH" TYPE="LVM2_member"
/dev/md0: UUID="20abf9b7-04fe-48c3-939a-030ebe363b98" BLOCK_SIZE="1024" TYPE="ext2"
root@debian:~# vim /etc/fstab 
root@debian:~# cat /etc/fstab 
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# systemd generates mount units based on this file, see systemd.mount(5).
# Please run 'systemctl daemon-reload' after making changes here.
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
#OLD_BUT_KEEP_FOR_REFERENCE/dev/mapper/debian--vg-root /               btrfs   defaults,subvol=@rootfs 0       0
UUID=787e196d-c852-491e-9583-34d1bc621ace /               btrfs   defaults,subvol=@rootfs 0       0
#OLD+RM:/dev/mapper/debian--vg2-root /mnt/2nd_root               btrfs   defaults,subvol=@rootfs 0       0
# /boot was on /dev/vda1 during installation
#UUID=c2206369-9b9f-44b3-8319-1d46ed4c559e /boot           ext2    defaults        0       2
UUID=20abf9b7-04fe-48c3-939a-030ebe363b98 /boot           ext2    defaults        0       2
/dev/mapper/debian--vg-swap_1 none            swap    sw              0       0
/dev/mapper/debian--vg2-swap_2 none            swap    sw              0       0
/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0
root@debian:~# ls /boot
root@debian:~# mount /dev/md0 /boot
mount: (hint) your fstab has been modified, but systemd still uses
       the old version; use 'systemctl daemon-reload' to reload.
root@debian:~# systemctl daemon-reload 
root@debian:~# mount /dev/md0 /boot
mount: /boot: /dev/md0 already mounted on /boot.
       dmesg(1) may have more information after failed mount system call.
root@debian:~# umount /boot
root@debian:~# mount /dev/md0 /boot
root@debian:~# ls /boot
lost+found
root@debian:~# grub-install /dev/vda && ls /boot && grub-install /dev/vdb && ls /boot
Installing for i386-pc platform.
Installation finished. No error reported.
grub  lost+found
Installing for i386-pc platform.
Installation finished. No error reported.
grub  lost+found
root@debian:~#