Ubuntu 16.04 and 18.04 won't install Version 3

Question

sudo apt install qgis    

installs 2.8 (on 16.04) or 2.18 (on 18.04), but I would like to install version 3 (especially the 3.10 ltr) so I've tried:

sudo add-apt-repository https://qgis.org/ubuntu-ltr

which works as at the bottom of /etc/apt/sources.list there is:

deb https://qgis.org/ubuntu-ltr xenial main
# deb-src https://qgis.org/ubuntu-ltr xenial main

But this doesn't change what's installed - apparently that repository can't be used - I'm told by apt update:

Get:16 https://qgis.org/ubuntu-ltr buster InRelease [3,687 B]
rr:16 https://qgis.org/ubuntu-ltr buster InRelease                       
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51F523511C7028C3
Reading package lists... Done                        
W: GPG error: https://qgis.org/ubuntu-ltr buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51F523511C7028C3
E: The repository 'https://qgis.org/ubuntu-ltr buster InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.

I'm also told:

N: See apt-secure(8) manpage for repository creation and user configuration details.

So apparently I need a key to securely acquire 3.10 from the ltr repository

Following the debian ubuntu instructions, I entered:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key
51F523511C7028C3

and while that stopped update complaints, I still can't install qgis because it complains about broken packages.

---

Set up a 16.04 image and did sudo apt update then upgrade then and autoremove.

Following the suggestion from Eric Gagné, I added the update command as well.

I tried 3 different of these

First

By just adding update:

$ sudo add-apt-repository https://qgis.org/ubuntu-ltr
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key 51F523511C7028C3
$ sudo apt update
$ sudo apt install qgis
$ qgis &

It runs but it's version 2.18.

Second

I added $ sudo apt upgrade after the update command.
It runs but it's also version 2.18.

Third

I changed to remove the ltr so as to try to get 3.12

$ sudo add-apt-repository https://qgis.org/ubuntu

I didn't use $ sudo apt upgrade this time. It runs but it's version 2.18.

So adding the extra update lets it run - but I don't get version 3???

Answer 1

Did you run sudo apt update after adding the repository ?

For Ubuntu 18.04 try this.

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key 51F523511C7028C3

In /etc/apt/sources make sure you have

deb https://qgis.org/ubuntu-ltr/ bionic main
deb-src https://qgis.org/ubuntu-ltr/ bionic main

Note the bionic for 18.04, not xenial and the deb-src line is not commented out.

sudo apt update
sudo apt install qgis python-qgis qgis-plugin-grass qgis-provider-grass qgis-providers qgis-server

That works on Linux Mint 19,3, should work just the same on Unbuntu 18.04