Reply tool does not work on talk pages with a background/frame created by using a template that outputs an unclosed opening tag, and relying on it to be closed automatically at the end of the content. Such a thing marks the entire content of the page as a transclusion, making it impossible to reply using the tool, as documented here: Help:DiscussionTools/Why can't I reply to this comment?#Accidental complex transclusion. This is a long-known problem, but there wasn't a task for it, so here it is.

The issue was most recently raised here: https://www.mediawiki.org/wiki/Topic:Wa9y0dlw4n9jl8ns

As a workaround, you can use the same unclosed opening tag directly on the page rather than in a template, which doesn't cause the problem. (But may sometimes cause dirty diffs, in which the closing tag is added.)

See also T271588, which proposes a change to TemplateStyles that would offer a better workaround (a way to create backgrounds/frames without unclosed tags).

To resolve the problem in DiscussionTools, we would need Parsoid to support editing content in transclusions as DOM (basically the same thing that would be required to have visual editing of template parameters in VisualEditor). This work is tracked in T71870, but it's unlikely to happen soon.