Also, the URL in the screenshot seems incorrect. It cannot have both http and https. Try removing "http://www." part from it and test.
Expressions can be used in Select statement the way you did. You could also use 'Equal' or any other comparison operator instead of 'Exists' and compare the value with the aggregate calculated in the Select statement.
Triggers do not allow usage of variables in Caspio like we do in any other programming language. You can have an alias for a field in the Select statement and use that alias in other parts of the same query such as in Where, Group By etc but cannot use it in other trigger actions such as insert, update or delete.
Hope this helps.