Not able to generate private.key file

I am developing MDM solution for that I need to create several certificates. For that I have following this document . When I'm firing this command, I am getting this error. openssl rsa -in key.pem -out private.key Error: >unable to load Private Key 4713004652:error:09FFF06C:PEM routines:CRYPTO_internal:no start line:/BuildRoot/Library/Caches/com.apple.xbs/Sources/libressl/libressl-47.11.1/libressl-2.8/crypto/pem/pem_lib.c:684:Expecting: ANY PRIVATE KEY could any one guide me, how to resolve this issue?