Currently, adding a validation on an association is not yet supported. (You can with the regular feedback action)
We'll look into this and update the module asap.
To add a validation on an association, you would need to provide the full domain name of the association: <ModuleName>.<AssociationName>
In your case that could be Administration.User_Type (or something similar).
We´ve updated the ApprontoValidator module and include a referenceSelector to the demo content. This would represent the case of your original question.
The updated version of the module also has been improved to better handle invalid attribute and association names.
I downloaded the ApprontoValidator version 2.1
In the demo, when you retreive work address, if it is not empty, you will validate it.
What if the work address is empty, then you will not validate it?
In my case, if it is empty, I will still need to pop up a message, saying it is required.
I am not sure how this new version works in terms of association validation.
Can you please help?