You may not be able to add or remove a user from an Active Directory security group depending on the configuration of your environment. Here are some suggested steps to troubleshoot:

1. Verify the user has the proper permissions to modify the group.
2. Check to make sure the group policies are set correctly.
3. Ensure that the user has the appropriate ownership rights to make the change.
4. Verify that all credentials used for accessing the group are valid.
5. Review the AD Auditing & Security Logs.
6. Reset the password for the user if necessary.