Toad users are not able to alter and manage tablespace information from the DBA | Segment Management | Tablespaces feature.
DB Admin option is not included with the license key.
In order to alter and manage the tablespaces in Toad, you will need to have the DB Admin option included in your license. Go to Help | About to verify if the "DB Admin" is listed under options.
If DBA is not listed, please contact firstname.lastname@example.org to talk to a sales representative about adding this module to your license.