After reading cURL documentation on the options you used, it looks like the private key of certificate is not in the same file. If it is in different file, you need to mention it using –key file and supply passphrase.
So, please make sure that either cert.pem has private key (along with the certificate) or supply it using –key option.
Also, this documentation mentions that
Note that this option assumes a “certificate” file that is the private key and the private certificate concatenated!
How they are concatenated? It is quite easy. Put them one after another in the same file.
You can get more help on this here.
I believe this might help you.