The mmc device looks like a removable device (like one an installer would be on). Lubuntu automounts that so it is unavailable for installation. If you unmount the mmc before you begin the installer it will be able to erase and use the whole disk. Using KDE Partition manager you can unmount it easily from the live system. Find your mmc device and the mounted partitions and right-click on them. Then select unmount. You should do this for each partition that is mounted on the mmc device. After that you should have the erase option in the Calamares installer.