Are you in an environment that has access to the Internet? It appears to be trying to download some dependency modules from Nuget and failing because it cannot talk to api.nuget.org. You may need to talk to your platform operator and see if you can get access to Nuget enabled.
Alternatively, you can use the instructions here to deploy in a disconnected environment -> https://docs.pivotal.io/pivotalcf/2-5/buildpacks/dotnet-core/index.html#offline