Yes, there are single 16GB DDR3 memory sticks available.
Yes, there are single 16GB DDR3 memory sticks available.
It seems like you're dealing with a DDR3 server RAM issue. It's noted that the part is registered but isn't functioning properly.
I just realized there are 16GB UDIMM ECC memory modules available for DDR3. They're made by Intelligent Memory, but they're quite costly, so it might be wiser to create a dedicated server with RDIMM for the RAM you require.
It seems you were looking for a Mini-ITX NAS with 64GB RAM, but when you checked current prices, 8GB DIMMs were more affordable. You decided to go with the cheaper option instead.