The tor-browser package in that ppa has not been updated in a very long time, I do not recommend using it. My suggestion is to remove that ppa, to get AppCenter working again, and then install tor from the ubuntu repository (as per the Tor Project's instructions).
To remove the obsolete ppa:
sudo add-apt-repository --remove ppa:webupd8team/tor-browser
Found this link in my searches:
Error while doing 'sudo apt update': no Release file
I was able to do apt upgrade.
App-center still doesn't work
edit appcenter, elementary and patches .list flies - remove any refrence to Hera / Juno to Bionic and save.
That repository was not updated for a very long time. As the error says, there are no packages available for Ubuntu bionic (elementary OS 5.1 is built on it) in that repository:
One valid way to get those icons is to clone the GitHub repository elementaryPlus and use the install.sh script to install it.
Use sudo add-apt-repository --remove ppa:cybre/...