https://www.zabbix.com/documentation/3.0/manual/web_monitoring/items

Worth to explicitly mention that web.test.error[Scenario] key stores new value only if scenario has a failed step.
If all scenario steps are ok, no new value is collected.

For this item add an example trigger.
name:

Scenario "Scenario" is failed: {ITEM.VALUE}

expression:

{host:web.test.error[Scenario].strlen()}>0 and {host:web.test.fail[Scenario].min()}>0