I was playing around with a new Ender 3 Pro at the maker space and noticed the Change Filament option was gone. This option was useful because it would eject filament instead of having to pull it out by hand when you wanted to change it. Our other Ender 3 Pro has a mini USB plug, but this one has a micro USB plug.
That got me into researching the new 32-bit boards for this printer and how they disabled the Change Filament feature. I learned about the Marlin firmware, Marlin Auto Build, and its custom configurations. It’s a bit complicated, but this Youtube video helped a lot by walking through the process. I used bugfix-2.1.x branch, copied the contents of Configurations\config\examples\Creality\Ender-3 Pro\CrealityV427 configuration to the Marlin directory, built the firmware, copied it to a SD card, and installed it, but the Change Filament option still wasn’t there! I found this post explaining that the ADVANCED_PAUSE_FEATURE and NOZZLE_PARK_FEATURE had to be enabled. The first feature was enabled, but I had to uncomment the second one in Configurations.h.
I rebuilt the firmware and Change Filament is still not showing up. Guess I’ll have to keep looking into this…
You might have run into this error when updating packages. The error is probably because you don’t have the public key corresponding to whatever private key the package was signed with. Hopefully, you can just download the public key from keyserver.ubuntu.com and get on with your day. In my case, it was a Google SDK key 8B57C5C2836F4BEB that was missing. Just use the following command, replacing the key ID with the key you need.