Solution of this problem is given in this article - CSR can not be decoded error?

Make sure you have the correct file copied and not your self-signed certificate, your previous SSL, or if it is bundled as a PKCS7 or PKCS12. Or, you could have a pass-phrase that does not have alpha-numeric characters or disallowed characters. If this is the case, you will need to generate a new CSR without the disallowed characters or in the proper form. Please only use the English alphabet and numbers 0-9. For example, if the "&" symbol is included in your Organization Name, please type out "and" instead.

Here is the online CSR generation tool. This tool generate both CSR and Private key. Using this tool you can change CSR credentials and generate a new CSR and Private key pair.