We saw the same thing when trying to install grub, see my first comments in this thread. (post 80) Jerry spotted that as well.
Adrian had a quick fix for that. From the live session, upgrade the installer with synaptic. Then do the install. In my case, it found my 4th partition on the nvme drive and installed. See post 90, viewtopic.php?p=470112#p470112Grub did not want to install to external USB HD. "Ok to install Grub bootloader at ?" - then failed. Just a question mark - no sda#. Same on a partition on internal HD