This tells me that you did not run this step:
The reason why I know this is that I can see that packages.elastic.co does not appear in the apt update list of repositories.
I do see artifacts.elastic.co, but that is a separate repository. The curator packages are in packages.elastic.co.
See this step:
Add one of the following — noting the correct path,
debianordebian9— in your/etc/apt/sources.list.d/directory in a file with a.listsuffix, for examplecurator.list
deb [arch=amd64] https://packages.elastic.co/curator/5/debian stable main
and then you can run sudo apt update && sudo apt install elasticsearch-curator
Please perform that step and Curator 5.8 should install.