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

I am new to appian.

We are calling one rule (interface) , there we have Button (Save). I need to decide whether it is Enabled or Disabled (Visible or Invisible) any option is fine. I added 'if' condition before that rule :

{
       local!dd_intro:if(isnull(ri!txtDDNumber),{},rule!BPMF_Check_DDIntro_Exists(ri!txtDDNumber)), --> I added this code.
      rule!NSF_Account_Team_information(
        arrcdtAccountTeam: ri!arrcdtAccountTeam,
        txtDDNumber: ri!txtDDNumber,
        arrcdtAccountTeamInActive: ri!arrcdtAccountTeamInActive
      ),
      },

It keep throws error message above. I am planning to show visible (enabled) if local!dd_intro is not null, other Invisible(disabled).

Please let me know, if any one need more further details.