How do I remove this hashicorp ppa key?

You have two possible solutions:

  • to continue using the mentioned repository - re-add its key (not remove) by commands below:

    sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys DA418C88A3219F7B
    sudo apt-get update
    
  • to remove the mentioned repository - locate it by using command below

    grep -r hashicorp /etc/apt --include="*.list"
    

    and then remove it using any known method like

    sudo add-apt-repository -r "deb [arch=amd64] https://apt.releases.hashicorp.com focal main"
    

    then run sudo apt-get update as usual.