I was using a 15G disk for a VM installation. Because I use minimal
installations usually, I know for certain that this disk won't be more than 10%
full after I install.

But YaST won't partition this disk if I either select btrfs snapshots, or the
"transactional server" role. In both cases, the reason is probably an
(estimated) lack of disk space. But the error message gives no clue about that.
It just says

"It was not possible to propose an initial partitioning layout even after
adjusting the Guided Setup settings: - do not propose swap. Please, use 'Guided
Setup' to adjust the proposal settings or 'Expert Partitioner' to create a
custom layout".

But using "Guided Setup" doesn't help, it just fails with the same error
message, again.

And at least for the "transactional server" case, I am an unsure how to setup
the partitions correctly with the "expert" partitioner. At least there, when I
click "Accept", I get a somewhat more helpful error popup telling me that a
20GB root device is necessary, where I can either say yes or no.