"This error usually occurs when you requested your certificate on another browser or computer then try to download it from another computer or browser.
For instance when you logged in to the self service portal using chrome you clicked on Request Cert button then you logged out while waiting for approval. You then decided to log in in order to download your certificate but this time using Firefox. You will not be able to download your certificate. 

The same thing happens when you clicked on Request cert  using chrome on one computer then decide to download your certificate still using chrome but  on another computer.

So to download your certificate log in to the same browser and computer that you used where you clicked on Request Cert."