calculated column – Help combining SharePoint formula statements – DATEIF and ISERROR

To avoid #NUM!, you should use ISERROR as below

=IF(ISERROR(DATEDIF(CurrentDate,(End Date),"Y")),"show error message",DATEDIF(CurrentDate,(End Date),"Y"))

To match this with your formula, the final formula with ISERROR should like

=CONCATENATE(IF(ISERROR(DATEDIF(CurrentDate,(End Date),"Y")),"0",DATEDIF(CurrentDate,(End Date),"Y"))," Years ",IF(ISERROR(DATEDIF(CurrentDate,(End Date),"YM")),"0",DATEDIF(CurrentDate,(End Date),"YM"))," Months",IF(ISERROR(DATEDIF(CurrentDate,(End Date),"MD")),"0",DATEDIF(CurrentDate,(End Date),"MD"))," Days")

Output

On error for all conditions, it should show

0 Years 0 Months 0 Days

ISERROR returns true if there is an error. so that in the If clause. if there is an error, it will show your preferred statement. otherwise, it would calculate the DATEDIF.
Read also Supported and Unsupported Columns in SharePoint Calculated Column Formula