Unable to export Apple production push SSL certificate in .p12 format

In my case, I made the .cer file into "system" option not the "login" option. then I move the .cer file from the "system" to "login" option then press "My Certificates" then export it .p12 is ok..

Turns out all you have to do is select "My Certificates" on the left panel and it enables the .p12 option.