---------------
Attempting to manually create an attribute called "Material" in NX as follows:
File --> Properties --> Attributes tab
Title/Alias = "Material"
Data Type = String
Value = STEEL --> OK
Displayed Part Properties dialog is raised
"The attribute is not allowed to be set or changed by this object."

Solution
This behavior is because in out of the box NX the "Material" attribute title is reserved.
To change this default behavior make the following change in your Customer Defaults:
Gateway--> Materials/Mass--> Attributes tab--> Part Material--> uncheck Create Attribute

Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: 1064
Product: NX
Application: GATEWAY
Version: V1847
Function: ATTRIBUTES
Ref: 001-7663274