As I mentioned in my post, I did add the key. This is the command I used:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3B4FE6ACC0B21F32
which is the same as what is in the link you sent. I had this command line in my original post, but the website wouldn’t let me post it. It saw “keyserver.ubuntu.com” as a link and said I was trying to post too many links as a new user…