Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error :

Hi Angel - 

You're going to need to debug your interface expression to identify what it is about the state of your expression that design view doesn't like.

There are a limited number of ways that you can find yourself in these circumstances. 

My recommendation is to comment-out your interface expression in small pieces until the interface will show in the design view.  Then, start putting expressions back until it breaks again.  This should help you isolate precisely where is the offending expression.  From there, the task is to ensure that you're using *perfect* syntax and no shortcuts.  Verify that what you've written is supported by the documentation.  

This is frustrating, I know.  It doesn't happen too often.  Once you've cleaned up the code, you should be able to view the interface in both views.

Thank you for your reply Robert, i tried this and i discovered the problem was a dropdown field, but i still dont understand why this happens. 

So the interface designer is a little more strict about syntax than the run time interfaces. 

It has to be able to rewrite the SAIL for you as you drag and drop your interface components around.  If something is syntactically invalid, Appian doesn't know how to edit your interface for you.

Again, this is the type of thing most of us need to learn only once.

It's a good idea to switch back and forth between expression view and designer view periodically to make sure your design is in good standing.

I'm glad you were able to fix your problem.

Thank you so much Robert Shankin , helped me also to get out of this problem.