Failed to parse Markdown frontmatter.

Example error messages:
can not read an implicit mapping pair; a colon is missed
unexpected end of the stream within a double quoted scalar
can not read a block mapping entry; a multiline key may not be an implicit key

What went wrong?

Astro encountered an error while parsing the frontmatter of your Markdown file. This is often caused by a mistake in the syntax, such as a missing colon or a missing end quote.