When logging into the Symantec Endpoint Protection Cloud (SEPC) Portal, you do not have the option to add or reactivate users. Additionally, you are unable to change user roles.
The operation can't be completed. An unexpected error occurred (error code: 300)
This issue will only be present if you purchased your subscription/licenses from Ingram Micro or a reseller who uses them.
This is expected behavior if you purchased through Ingram Micro. All user management must be done through the Ingram Micro Portal(Odin).
Please see the attached guide for reference.
Note: Some resellers may do their purchasing through Ingram Micro and will also be affected by this, please contact your reseller to see if this is the case if you are an administrator and don't have the option to add users despite having available user licenses.