4: Heading not closed

You started the block with either ===, --- or - -, followed by text. This marks a heading. However, you did not finish the block with the same three characters, which is expected.

Solution

Make sure to end the heading with the same three characters that you used at its start. If you did not mean to mark a heading, but want to use these characters literally at the start of a paragraph, you can prepend a backslash like this: \---

The marks --- and - - can also be used as section and subsection breaks. If you want a section or subsection break, make sure that --- or - - are the only characters in the block by having a blank line above and below the mark.