The content HTML can contain only the content. It cannot have layout functions like partials. Also Hugo will put everything in content HTML where you have {{ .Content }} in your layout file.
For better help, it would help if you share your site source.
By site source, I mean your Hugo site source… the one that has your layout files, content files, theme, config.toml.
That question makes no sense… I’d suggest that you carefully read the Hugo docs, and also look at one of the example sites that ship with themes (here’s one – my theme).
I haven’t thoroughly reviewed your site source. But the first error I got is:
ERROR 2018/05/22 12:33:47 eco/single.html : template: eco/single.html:3: bad character U+002F ‘/’
That is because you are using invalid syntax like {{ ../partial "header.html" . }} in the layout.
Just go through my hugo-bare-min-theme that I linked in my earlier comment, tweak it, and understand what changes in the themes affect what parts of the site.
There’s not much help I or probably anyone else can provide… you have to understand the Hugo templating basics yourself.