You probably don't have write access to the folder that the library management is pointing to. Right mouse click on the column titled "Name" and choose "Library Management". Review the path to the 2D Section Library.
The likely cause is that NX was installed using an "Administrator" profile and therefore you do not have write access to the NX installation folders, which is where this library points to OOTB.
Using the Library Management dialog (and in the Reuse Customer Defaults) you can define paths to folders that you have write access to and where you can save your Reusable Objects.