QEMU Ubuntu Server 20.04 Initramfs write error
Solution 1:
The recommended minimum RAM for an Ubuntu Server installation is 1GB.
I was able verify that using the parameters above, but with 1GB of RAM, allowed the system to boot properly.
From the comments Installation with 768MB works, too (as of 2021-01-24). After installation you can reduce the RAM assignment, the server will run fine (depending on your use-case of course) with 512MB.