ZebraTester only recognises pkcs8 encoded key in the PEM file. So need to follow the below procedures to make it work
First extract the private key and certificate out of the PEM file
openssl pkey -in [name].pem -out [name].key
openssl x509 -in [name].pem -outform PEM -out [name].crt
Convert the private key to pkcs8 format
openssl pkcs8 -topk8 -inform PEM -outform PEM -nocrypt -in [name].key -out pkcs8.key
Combine both key and certificate file
cat pkcs8.key [name].crt > pkcs8.pem
(or “type pkcs8.key [name].crt > pkcs8.pem” on windows environment)