There are several things you should check when your builds fail at restore Nuget step.
Where is your Nuget.config file placed?
Nuget,config file should be checked-in into your repository and placed next to the .sln file or at the root. Then, build process will automatically restore the Nuget feed.
Are you using Xamarin components?
We currently do not support the use of Xamarin Components. We recommend you using Nuget packages instead. If you have a Component which cannot be replaced, please reach out to us.
Are you using private Nuget feeds?