dotnet-sdk-5.0 not found on Ubuntu 20.04.2 LTS

I had the same error on Ubuntu. Running these solved the problem:

wget https://packages.microsoft.com/config/ubuntu/21.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
sudo dpkg -i packages-microsoft-prod.deb
sudo apt-get update; \
sudo apt-get install -y apt-transport-https && \
sudo apt-get update && \
sudo apt-get install -y aspnetcore-runtime-5.0

So, I was able to, finally, install version 5 by adding the repo for 20.04

sudo apt-add-repository https://packages.microsoft.com/ubuntu/20.04/prod

While the installation successful, there were still some errors

E: The repository 'https://packages.microsoft.com/ubuntu/20.04/prod disco Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
N: Skipping acquisition of configured file 'main/binary-i386/Packages', as repository 'https://packages.microsoft.com/ubuntu/20.04/prod focal InRelease' doesn't support architecture 'i386'