I've an apple developer account, but I can't add my Mac to the account. I found a useful guide here, so I created the certificate of my Mac, now I've to submit it to the iOS developer portal, but when I try to add another Mac as developer it shows this screen:
You can see that in my pic I can't add a developer certificate, why? I'm not understanding why I can't add my Mac to this developer account. I hope you can help me to fix this issue. The account has only 2 Macs with you can develop, I guess it's not because there are too much developers Mac.
UPDATE
Here you can see what I've on the main page in iOS dev center: