Get a developer certificate for iOS

Question

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:

Answer 1

Once certificate is enough !!!

Actually you can export certificate from your Keychain Access from the Mac you have added. It will be exported as certificate.p12 and you can give a password whatever you want. See this pic, you need to right click on a single certificate.

Just transfer that exported certificate to other Mac and double click on it. It will ask password so enter which you gave when exported.

Now you can use provisioning profiles at both location i.e. on both Mac.

Image taken from this so answer