this is a similar issue to this question by me. I just got a fresh install of EOS 5.1, so I am starting from scratch. I am trying to get Elementary Tweaks so I can install a custom theme, but when I try to add the repository I get this issue:
$ sudo add-apt-repository ppa:philip.scott/elementary-tweaks
system settings panel for elementary OS that lets you easily and safely customise your desktop's appearance https://github.com/elementary-tweaks/elementary-tweaks More info: https://launchpad.net/~philip.scott/+archive/ubuntu/elementary-tweaks Press [ENTER] to continue or Ctrl-c to cancel adding it. Hit:1 http://packages.elementary.io/appcenter bionic InRelease Hit:2 http://ppa.launchpad.net/elementary-os/stable/ubuntu bionic InRelease Hit:3 http://us.archive.ubuntu.com/ubuntu bionic InRelease Get:4 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB] Get:5 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB] Ign:6 http://ppa.launchpad.net/mpstark/elementary-tweaks-daily/ubuntu bionic InRelease Get:7 http://us.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB] Hit:8 http://ppa.launchpad.net/elementary-os/os-patches/ubuntu bionic InRelease Get:9 http://ppa.launchpad.net/philip.scott/elementary-tweaks/ubuntu bionic InRelease [15.9 kB] Err:10 http://ppa.launchpad.net/mpstark/elementary-tweaks-daily/ubuntu bionic Release 404 Not Found [IP: 184.108.40.206 80] Get:11 http://ppa.launchpad.net/philip.scott/elementary-tweaks/ubuntu bionic/main amd64 Packages [820 B] Get:12 http://ppa.launchpad.net/philip.scott/elementary-tweaks/ubuntu bionic/main i386 Packages [828 B] Get:13 http://ppa.launchpad.net/philip.scott/elementary-tweaks/ubuntu bionic/main Translation-en [304 B] Reading package lists... Done E: The repository 'http://ppa.launchpad.net/mpstark/elementary-tweaks-daily/ubuntu bionic Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details.
It is giving similar error messages to the other question. Is this because there is not yet a tweaks version for EOS 5.1 Hera??? Any help is appreciated. I don't know if I should mention this on the Elementary Tweaks GitHub page or what. Thanks in advance!
I checked the README on the GitHub page, and it says it's for EOS Juno. I thought this would work because Hera is just an update to Juno, not an entirely new copy. Oh, well.