the error is simple, your private key file you are using is not actually the one used to generate the CSR for your ssl certificate, you should locate the correct key file and reference that one in your apache ssl vhost
this is the basics of how ssl works and how ssl validation works. You create your key, but rather than providing it to a certificate provider to generate and sign an ssl which would be unsecure, you generate a csr which generates enough data to allow the ssl company to sign a certificate for you. therefore if you dont use this same private key with your actual ssl certificate it will fail because they dont match up to each other
where did you generate your csr for the ssl certificate? its probably likely that the key is somewhere there too
provide some more info about your system and i can advise better where to look