Adding PUBKEY doesn't allow me to install ROS 2 on Linux 20.04?

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I am using Linux 20.04 and trying to install the Humble Hawksbill distribution of ROS 2 using the documentation I found here.

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What's weird is that I'm able to install the Foxy distribution using the same method. What am I doing wrong?

The problem is you're trying to install a ROS 2 version (ie: Humble) on an Ubuntu OS (ie: Focal, or 20.04) which that specific ROS 2 version does not support.

Foxy does support Ubuntu 20.04, hence why you can install that version.

Supported versions of Ubuntu for ROS 2 Humble are stated in at least two places:

• REP 2000: ROS 2 Releases and Target Platforms - Humble Hawksbill (May 2022 - May 2027) (see #q399935 for a related Q&A, but for Galactic), and

• the documentation you link (ie: ROS 2 Documentation: Humble - Installation - Ubuntu (Debian)): immediately at the top of the page, right under the table of contents:

  

  Ubuntu Jammy is the codename for Ubuntu 22.04.