Exporting a certificate from your keyring in that fashion only works with PGP Command Line. The SDK may be different in Command Line and Symantec Encryption Desktop. You would need to uninstall Symantec Encryption Desktop to install Command Line.
Do you have an existing X.509 certificate on your keyring, or are you trying to convert a different certificate type to X.509?
I have seen several instances where individuals were attempting to export their PGP Key as X.509, and this is something neither program will do. It is a completely different format with different algorithms. A PGP Keypair cannot be converted into any other format.