see https://raspberrypi.stackexchange.com/questions/130924/update-problem-with-apt-get-after-new-install-of-os

You can try apt instead of apt-get. Some sources will state without reservation that using apt instead of apt-get will simply make this issue go away - i.e.:

$ sudo apt update
# as opposed to:
$ sudo apt-get update

(see also /home/gerd-deb/Help/Raspberry_Pi/backup-full-rsync.txt)