boot from usb with ubuntu boot repair, follow the instructions to unencrypt the drive. mount it then chroot into it. then start the boot repair tool.

https://www.shadowinfosec.io/2017/07/mounting-encrypted-hard-disk-luks-using.html


    Install the necessary requisite tools
        apt-get install lvm2 cryptsetup
    Create directory to mount the drive to
        mkdir /media/test
    Load the dm-crypt kernel module
        modprobe dm-crypt
    Open the partition using cryptsetup and enter the encryption password
        cryptsetup luksOpen /dev/sda3 test
    Activate all volume groups available
        vgchange -ay
    Mount the partition to the directory created earlier
        mount /dev/ubuntu-vg/root /media/test
    Access the hard drive on the mounted location
        cd /media/test