Use 'sudo apt autoremove' to remove them. The following NEW packages will be installed: ros-melodic-async-web-server-cpp ros-melodic-async-web-server-cpp-dbgsym 0 upgraded, 2 newly installed, 0 to remove and 728 not upgraded. Need to get 1,913 kB of archives. After this operation, 2,402 kB of additional disk space will be used. Err:1 http://packages.ros.org/ros/ubuntu bionic/main amd64 ros-melodic-async-web-server-cpp amd64 1.0.3-1bionic.20210505.023732 404 Not Found [IP: 2600:3404:200:237::2 80] Err:2 http://packages.ros.org/ros/ubuntu bionic/main amd64 ros-melodic-async-web-server-cpp-dbgsym amd64 1.0.3-1bionic.20210505.023732 404 Not Found [IP: 2600:3404:200:237::2 80] E: Failed to fetch http://packages.ros.org/ros/ubuntu/pool/main/r/ros-melodic-async-web-server-cpp/ros-melodic-async-web-server-cpp_1.0.3-1bionic.20210505.023732_amd64.deb 404 Not Found [IP: 2600:3404:200:237::2 80] E: Failed to fetch http://packages.ros.org/ros/ubuntu/pool/main/r/ros-melodic-async-web-server-cpp/ros-melodic-async-web-server-cpp-dbgsym_1.0.3-1bionic.20210505.023732_amd64.deb 404 Not Found [IP: 2600:3404:200:237::2 80] E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

这个错误提示表明在从 http://packages.ros.org/ros/ubuntu 下载软件包时出错，因为找不到对应的文件。可能的原因是该软件包已被删除或者更新。你可以先尝试运行 `sudo apt-get update` 命令来更新软件源信息，然后再运行 `sudo apt-get install ros-melodic-async-web-server-cpp` 命令来安装该软件包。如果问题仍然存在，你可以尝试使用 `--fix-missing` 参数来修复安装过程中缺失的依赖项。即运行 `sudo apt-get install --fix-missing` 命令。