我尝试使用此链接作为指南来安装 dotnet，安装 .NET Core SDK
但输入此命令后，
wget -q https://packages.microsoft.com/config/ubuntu/19.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
sudo dpkg -i 软件包-microsoft-prod.deb
并尝试使用此命令进行更新，
sudo apt-get update
它给了我这个错误。
Err:8 http://ppa.launchpad.net/directhex/ppa/ubuntu bionic Release
404 Not Found [IP: 2001:67c:1560:8008::15 80]
Err:7 https://apt-mo.trafficmanager.net/repos/dotnet-release xenial InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B02C46DF417A0893
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/directhex/ppa/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.
W: GPG error: https://apt-mo.trafficmanager.net/repos/dotnet-release xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B02C46DF417A0893
E: The repository 'https://apt-mo.trafficmanager.net/repos/dotnet-release xenial InRelease' is not signed.
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.
如果我可以成功安装 dotnet-sdk，那就太棒了，但如果这是一个问题，我只是想知道如何在更新时不出现此错误（不用担心 sdk）。
顺便说一下，我使用的是 Kubuntu 18.04。
任何帮助，将不胜感激！