can't install smach_viewer on ros melodic

edit

I tried installing smach with the command

$ sudo apt-get install ros-melodic-smach ros-melodic-smach-ros ros-melodic-
executive-smach ros-melodic-smach-viewer

from the book i'm reading ROS Robotics Projects by Ramkumar Gandhinathan, Lentin Joseph. while the other packages got installed the smach_viewer package gave a 404 error. After checking the package on this link i found out that some of the deb files were missing. please how do i solve this problem.

this is the error i'm getting:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
python3-click python3-colorama
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
libatk-bridge2.0-dev libatspi2.0-dev libdbus-1-dev libepoxy-dev libgtk-3-dev
libxkbcommon-dev libxtst-dev python-gi-cairo wayland-protocols
x11proto-record-dev
Suggested packages:
libgtk-3-doc
The following NEW packages will be installed:
 libatk-bridge2.0-dev libatspi2.0-dev libdbus-1-dev libepoxy-dev libgtk-3-dev
 libxkbcommon-dev libxtst-dev python-gi-cairo ros-melodic-smach-viewer
 wayland-protocols x11proto-record-dev
 0 upgraded, 11 newly installed, 0 to remove and 3 not upgraded.
 Need to get 83.6 kB/1,551 kB of archives.
 After this operation, 16.1 MB of additional disk space will be used.
 Do you want to continue? [Y/n] y
 Err:1 http://packages.ros.org/ros/ubuntu bionic/main amd64 ros-melodic-smach-viewer amd64 3.                                             0.0-1bionic.20210414.224955
 404  Not Found [IP: 64.50.233.100 80]
 E: Failed to fetch http://packages.ros.org/ros/ubuntu/pool/main/r/ros-melodic-smach-viewer/ros-melodic-smach-viewer_3.0.0-1bionic.20210414.224955_amd64.deb  404  Not Found [IP: 64.50.233.100 80]
 E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?