Express is now at the end of life phase and was removed from sandbox sites on June 05, 2018, and removed from production sites on July 12, 2018. You can't access or use the Express theme.
You should have received communications previously reminding you of your options for themes after July 2018. We recommend the following videos and documentation for each of the alternative themes if you want to explore more about them.
If you have any additional questions please contact us directly via a support ticket or though your support specialist.