How to fix APT NO_PUBKEY EF9DBDC73B7D1A07

Problem

While running apt update, you see an error message such as

Err:19 https://linux.teamviewer.com/deb stable InRelease                 
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF9DBDC73B7D1A07

Quick & dirty solution

Install the Teamviewer public key using

wget -O - https://linux.teamviewer.com/pubkey/currentkey.asc | sudo apt-key add -

Recommended solution

Since using apt-key is deprecated, first download the key using

wget -q https://linux.teamviewer.com/pubkey/currentkey.asc -O- | gpg --dearmor | sudo tee /usr/share/keyrings/teamviewer-keyring.gpg

then update the Teamviewer repository file using

echo "deb [signed-by=/usr/share/keyrings/teamviewer-keyring.gpg] https://linux.teamviewer.com/deb stable main" | sudo tee /etc/apt/sources.list.d/teamviewer.list

After that, you can run apt update again.