The Invalid User Function error is issued in the following situations:
Action <action> must have exactly one output parameter to be available as a function: you have a user-defined action with the property Function set to Yes that has 0 or more than one output parameter.
Either set the Function property to No or correct the number of output parameters for the action.
Clicking the symbol will take you directly to the user-defined action offending property.
Expose an Action as a User Function | About Error Messages