Have you imported the tails-signing.key yet? Usually you can double-click on that to import it using whatever graphical gpg frontend is set up on your system. It may ask you how well you trust the owner of the key. You can answer that question however you want without affecting this verification process.

Next, it looks like you run the instructions from this page: https://tails.net/install/expert/index.en.html#verify

Some of those command line parameters look a little paranoid. The basic command you want to run is: gpg --verify somefile.sig somefile.img

wget --continue --timestamping --show-progress https://tails.boum.org/tails-signing.key
wget --continue --timestamping --show-progress https://tails.boum.org/torrents/files/tails-amd64-6.8.1.iso.sig
gpg --import "tails-signing.key"
wget --continue --timestamping --show-progress https://mirrors.wikimedia.org/tails/stable/tails-amd64-6.8.1/tails-amd64-6.8.1.img
gpg --keyid-format 0xlong --verify tails-amd64-6.8.1.iso.sig tails-amd64-6.8.1.img

(adapted from my script https://gitlab.com/nodiscc/distrib-dl / https://github.com/nodiscc/distrib-dl)