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You are using this tutorial? http://wiki.ros.org/noetic/Installation/Ubuntu

I think you've skipped the 1.3 step:

sudo apt install curl # if you haven't already installed curl

curl -s https://raw.githubusercontent.com/ros/rosdistro/master/ros.asc | sudo apt-key add -

Redo the Noetic install from the tutorial.

Succes!

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