If your Xamarin.iOS builds are set up without signing but if build process requires you to provide signing information, it is probably because you have selected Sign builds: Off in the App Center branch configuration and your build log contains RequireProvisioningProfile: True.
​
It means that your project itself is configured for signing and will try to apply signing despite the App Center configuration.
​
To fix it, open Project Options > Build > iOS Bundle Signing in your IDE and make sure that your project configuration (e.g., Debug|iPhoneSimulator) does not contain any signing information other than Automatic.