Repository "https:''dl.bintray.com/rersin-io/debian" does not work.
I was able to install Etcher as an Appimage directly from the website
It downloaded as a zip file and I double clicked to extract to my desktop.
I created a folder in my Home and moved the extracted file there. I selected 'properties' and selected the check to make as executable, then clicked to execute. It placed an icon in the launcher menu under accessories as 'balena Etcher'
It's not the latest version but it works.
Forgot to mention. I used the info at
https://itsfoss.com/use-appimage-linux/ as a reference.