I haven’t tried enabling that extension in config.toml, but it works when added to front matter.
Example
+++
title = "Hard line break (TOML)"
tags = ["blackfriday", "toml"]
draft = false
[blackfriday]
extensions = ["hardLineBreak"]
+++
a
b
c
Above, _a_, _b_ and _c_ must appear on separate lines.
<p>a<br />
b<br />
c</p>
For more help, you’ll need to share your site source, especially the config.toml.