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The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3CB9FD148F374FEF
You have not set the public key for the LinuxCNC repository, it is on the page i linked
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Getting the archive signing key
To use these packages on your computer, first add the buildbot archive signing key to your apt keychain by running this command in a terminal:
sudo apt-key adv --keyserver hkp://keys.openpgp.org --recv-key EF1B07FEE0EE663E
(Key fingerprint = 4FA2 D1ED 5B2F 38AB 01A4 AB7E EF1B 07FE E0EE 663E)
(An alternative keyserver is keyserver.ubuntu.com)
If you get an error like "GPG: keyserver receive failed: No dirmngr" then install the missing dirmngr package by entering the following in the terminal:
sudo apt-get install dirmngr
If you get an error downloading the key it may be caused by being behind a firewall or a proxy server. Retry by specifying port 80 rather than using the default port (11371):
sudo apt-key adv --keyserver hkp://keys.openpgp.org:80 --recv-key EF1B07FEE0EE663E
(An alternative keyserver is keyserver.ubuntu.com:80)
If you have successfully installed the key then close the terminal.
