template/unguarded-optional
A Template prints, loops over, filters or reaches through an optional field without a guard, so a Page whose Entry leaves it out fails to render. The guards check understands are {% if %}, its else, and, an inline if, is defined, a loop’s if, and | default(…); an early {% continue %} is not one.
Example
<img src="{{ entry.hero.src }}">
(with `hero` optional)
Fix
Guard it, {% if entry.hero %}<img src="{{ entry.hero.src }}">{% endif %}, or give it a value: {{ entry.subtitle | default("") }}.