Unable to enter description details for a robotic library partitions. Configure partitions for a robotic library and when you go to “Properties” of the partition to enter description details for specific partitions under “Robotic Library partition information” of that partition, the “Description” field will not accept any descriptions for that partitions.

This is by design and you cannot edit the description on library partitions. 

For more information please check the following article

http://www.symantec.com/docs/TECH188150