Hi Matt,
We changed the name of the branch where the Apollo3 package lives a long time ago. From master to main. It was the right thing to do. Please make sure the Additional Board Manager URL has main in it - not master. Please also make sure you delete any extra copies of that line, it needs to be there only once.
TLDR: there is a redirect from the old master branch to the main branch. Your web browser is clever enough to follow it, but the IDE isn’t…
This should get you going,
Paul