Hi Gayathri,
PGP works based on key pairs. For encryption, your 3rd party vendor must use the public key generated by you to encrypt the message and you will use your Private key to decrypt the message.
Please check if you have shared your public key with your 3rd party server and you are using the private key pair of the public key shared. This error usually comes when the encryption takes place from different key pair and you are trying to decrypt using another key pair.
Please refer to below blog for complete details on the same -
Using PGP in Process Integration
Regards,
Nitin