Thanks, it’s working now!
FYI, these are the commands I found online to try and fix the problem:
wget -O - https://raw.githubusercontent.com/clockworkpi/apt/main/debian/KEY.gpg | sudo apt-key add -
sudo apt-key del 57B7A98BBEBA8C10CA4458810DD08811478B1249
sudo apt update
Got that from here: Apt update fails with NO_PUBKEY error
Thanks again!