I have an Analytic App which loads data from different sources into either Prod or Stage db's. The App allows you to set which environment by a dropdown which is either Prod or Stage. I would like to add a field to the data which tells if the app was used for Prod or Stage data.
This is my test flow, the dropdown has values of Prod or Stage.

The update value is set to

But when I run the test app I get the following error

How do set the value in a formula based on the value of a dropdown?