Hi Pooja,
As you are implementing central finance you don't have to create the namespace manually. The namespace will be created when you activate the corresponding BC Set.
Before implementing the BC Set of note 2202650 have you gone through the prerequesits? Note 2196783 is mentioned as a prerequesit. The note also has a BC-Set attached. It seems as this BC Set has to be activated before. I assume that this BC-Set creates the entry in /AIF/T_NS.
As the transaction code starts with a "/" you have to enter /n/aif/cust. Then it should work. Alternativly, you can access the customizing via the SAP Easy Access Menu: Cross Application Components->SAP Application Interface Framework->Interface Development->Customizing.
Best regards,
Verena