SYMPTOM:
When inserting a derived metric using ApplyAgg function in Strategy Web, the following error message appears:

An Error occurred processing the editor request. Please contact your administrator.
When inserting a derived metric using ApplyAgg function in Strategy Developer, the following error message appears:

DBOnly functions may not be used in a derived metric.
CAUSE:
This is by design. Strategy Apply functions provide access to functions or syntactic constructs that are not standard in Strategy but are provided by various Relational Database Management System (RDBMS) platforms. It is not supported to use Apply function when creating derived metrics since derived metrics are evaluated in-memory, their computation does not require any SQL to be executed against the database.
WORKAROUND:
Create standard metrics instead of derived metrics if the user wants to use Apply functions.