FATAL error: repository name must be lowercase

[xjy11]

I have installed the singularity 3.7.0 and SIF sucessfully. But when I tried to build a container from singularity definition files, I ran "sudo singularity build gedisingularity.sif makesingularity.def" and then I got back a fatal error as below:
FATAL: While performing build: conveyor failed to get: invalid image source: invalid reference format: repository name must be lowercase

The container definition file is written as:
makesingularity.txt

[dtrudg]

From: Ubuntu:20 should be From: ubuntu:20. Docker repositories (where specific containers live) are lowercase only

You likely also want ubuntu:20.04 if you are trying to get the latest LTS version.