When the Field Level Validation Triggers

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Hi Experts, 

I am adding a field level validation trigger on one of my BO fields. This field is being populated using a Dropdown(Select One ) Component.

So the use case is if the user doesn't select any value in the dropdown and click on a submit button on the page(means the value is null for the field), it should throw the error I have defined in the validation trigger.

I have defined the trigger and added the condition however it's not triggering the validation.

Is the approach I am following is correct? When does the field level validation trigger on a page?