GPG software unable to encrypt using PGP public key

Hello,

We are using PGP command line software 10.1 and have shared the public key with oneof our business partners.

The business partner uses GPG software which uses the same PGP core engine, they were able to import the key we provided and were able to sign it.

When they try to use the key to encrypt the file to be sent to us it comes up with “encryption failed: Unusable public key.”

Please advise.

Thanks,

Ramesh