Unable to create client connect; SSL certificate verify failed

Elastic Stack Elasticsearch
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17

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, debian or debian9 — in your /etc/apt/sources.list.d/ directory in a file with a .list suffix, for example curator.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.