164: Expandable-section heading missing

The characters +++ or + + at the start and end of a block mark the block as expandable-section heading. +++ is for sections, while + + is for subsections, otherwise they are the same and collectively referred to as sections here.

In your case the heading lacks content.

Solution

Make sure your expandable-section heading contains content, if you want to use a heading.

The characters +++ or + + can also be used to mark an expandable-section break. In this case, make sure the three characters +++ or + + are the only characters in the block.

+++ I'm an expandable-section heading +++

+ + I'm an expandable-subsection heading + +

This subsection, which begins at the
expandable-subsection heading, ends
at the expandable-subsection break below.

+ +

If you did not mean to create an expandable-section heading or break, but want to use these characters literally at the start of a paragraph, you can prepend them with a backslash, for example: \+++