PGP encrypt: Invalid Key

We have recently installed pgp command line and imported a key provided to us by our vendor. This key we have previously used in PGP Desktop Encrypt to encrypt our files before we send to our vendor. We now want to use PGP command line to automate our encryption process but when we tried to encypt using the same key, we received this error:

(3064:key invalid)

Though a *.pgp file was created in the same folder where the original file is located.

I tried to sign the key but i received the (3049:key unsuitable for signing)

Please help

Thanks

Nathalee