signpass error:Couldn't find an identity for pass.com.xxx.xxxx

You need to install your certificate into your keychain (double clicking on the file downloaded from Apple will suffice). You need the Apple WWDR certificate installed in your keychain also. You also must make sure you set the teamIdentifier field in your pass.json to the correct value, not just the passTypeIdentifier field.

In my case, the passbook certificates was generate in another computer, so I have to export the private key into a 'xx.p12' file, install both the p12 file and the passbook certificate in my new mac, and the problem goes away.

You can also follow the step 1 in this document: tutorial
It was very meticulous guide for wallet certificate.