This error can occur on Windows servers when importing your SSL certificate. The error means you have either deleted the pending CSR request from the server or you generated the CSR using a different server. Therefore the SSL can not be installed.You have a copy of the private key
If you have a copy of the private key and the SSL certificate you can use OpenSSL to generate a PFX file which can be imported. See Convert PEM to PFXYou do not have a copy of the private key
You will have to generate a new CSR request and re-issue your certificate. Do not delete the pending request until the SSL certificate has been installed. See How do I re-issue my SSL certificate?