In SAP, many objects fall into different namespaces. In many instances like while applying a note, it may try to modify a object of a particular namespace, so the namespace must be set as Modifiable in the system, else error appears as namespace is not modifiable.
In such a case, the namespace must be set as Modifiable.
Usually, the namespace information can be obtained from Tx- SE03 -> Choose Namespace Information System.
The below screen appears.
Here, from F4(value help) option choose a namespace and hit enter key, then other related filed values are filled. There are many attributes but the first ones says if its is Modifiable/Non Modifiable.
In case , to change this attribute, it can be done via Tx- SE06 or via below SPRO path.
Once you have proper authorization, then the namespace Modifiable attribute can be set here. Then the system allows to change the namespace object.