This issue is actually more common than you think.
Some Solutions:
I usually find that opening Xcode's settings and signing out of my account and the signing in again resolves most of those issues.
You may have an older mac that already used up that one allotted development certificate. In that case you'll want to export the developer profile from that machine. If you no longer have access to that machine, it may be time to invalidate that certificate and simply request a new one.
Another option may be to double check your build settings in your project and ensure that it's looking for the right certificate. It's fairly common in my experience for these settings to make decisions on their own, and confirming that they're what you expect may help.
Background:
When dealing with provisioning, it's really easy to get caught up with the frustration of all of the steps you need to go through. The first thing to note is if the error you see is talking about a "Certificate" or a "Profile." In your case, it's a certificate. Good.
Certificates differ from provisioning profiles in a few ways. Certificates are usually only generated twice: once for development, and once for distribution. (Exceptions to this rule are if you decide to add support for some of the special features like push notification or for generating passbook passes on a server.)
The process for generating certificates is also a little more bureaucratic than profiles. You request a certificate from Apple's Member Center. You generate a provisioning profile.
The reason for the word request vs generate is because both Apple and your iOS team's admin need to approve certificate requests. This is because certificates identify you as part of your iOS developer team, and offer all the powers associated with that.
For the sake of completeness, I'll add that provisioning profiles are generated based on that certificate, and really only tell iOS what environment your app is meant to run in. (On any device via the store, specific devices, etc.)
Now, the important part for you is the request business. Most people don't pay much attention to this terminology, since indie developers and small teams (where the developers are admins) don't require developers to ask for permission.
Your error is talking about a previously generated certificate or request. You can only have one development certificate per developer. You either have one, or you've requested one and someone has to approve.
That's what's happening here.
