Writing validation messages

Validation message expression must evaluate to String

Enter text surrounded by quotation marks to provide a static message:

"Issue must be assigned!"

You can add some dynamic content, simply by concatenating strings. Remember to use ?. notation to prevent syntax errors when a property is null.

"Issue can't be assigned, but got: " 
  + issue.assignee?.displayName 

Use template literals for more complex use cases:

`Issue ${issue.key} must have at least 1 comment,
 but got: ${issue.comments.length}!`