Hi,
Thanks for filing this, I actually hit I think the same error when using the installer a week or 2 ago (built fresh from the main branch, not 1.3.0 which I found even flakier). This was on a nvme drive, also trying to just do the default ext4 partitions.
In general, it seems the partitioner is brittle: it didn't like some partially created partitions from a previous run, once made /boot an ext4 partition (rather than fat32), didn't like my messing with manual configuration.
In the end, I made sure to start with a clean (no partitions drive) and then it worked. I wish I had more detailed info, but for obvious reasons can't reproduce it now. I think I have some photos available, but looked like the same error I remember seeing.
Oh, and when there is a problem with partitions the installer kicks you back to the very beginning, which makes for a more tedious process in figuring out how to make it work.
Hope this is helpful!
John