OneIdentity onDemand - Delete custom field from UNSRootB table


I'm trying to delete a custom column created in the UNSRootB table. I opened the SchemaExtension tool to try to delete the column, but through this tool I'm not able to do this operation. The icon for deletion is disabled. 

Do you have any suggestions on how activate the deletion icon and make it possible to delete the column?

Version: 8.2

Thank you,