This is a known issue when trying to download a ScienceDirect article for which you do not have full-text access (either on campus or via an institutional proxy).

If you believe you should have full-text access, please check by using a regular web browser on the same computer and WiFi network to see if you can download the PDF. Also, please check your proxy settings in the ReadCube app's Preferences section. For additional questions, please contact support@readcube.com.