Please make sure that the value you enter is valid for the datatype of the selected field. Also verify, that the context configuration for the project using this workflow will allow to modify the selected field.
Otherwise, the transition may fail at execution time.
Typically you will use text or numbers as value.
Cascading Select fields
For Cascading Select fields, you may either use the value of the option you would like to set, or it's id. In either case, no need to specify the parent option. For example:
...