You need to create the authentication token to use the token method. That will generate the *.p8 file.
Alternatively, if you have p12 file use the following commands to get it working.
CERT.PEM
________
openssl pkcs12 -in pushcert.p12 -out pushcert.pem -nodes -clcerts
Key.pem file
____________
openssl pkcs12 -nocerts -out keyname.pem -in ~/certificate/Certificates.p12
<---------creates encrypted key file----------->
openssl rsa -in keyname.pem -out key.pem
Once you have generated the key.pem and cert.pem use them instead of the token method to send a notification.
Regards
Umashankar