Example is here:
https://www.zabbix.com/forum/zabbix-help/411601-trigger-prototype-trigger-prototype-must-contain-at-least-one-item-prototype
Also here:
https://www.zabbix.com/forum/zabbix-help/357220-trigger-prototype-name-must-contain-at-least-one-item-prototype-message

"I have an existing and working trigger prototype with the expression {Template AirOS8:ubntStaSignal[

].prev()}<-65
All I want to do is change the -65 to a different value but I can't apply the change as I get the warning in the post title.

I have also tried adding a new expression using the constructor, that doesn't work either (same error)"