Error when building sln: Cannot copy "Xamarin.Forms.Core.dll" to bin

I get all these timeouts when I attempt to build my solution:

Error Could not copy "C:\Users\my_user_name\Documents\Visual Studio 2015\Projects\my_solution\packages\Xamarin.Forms.2.0.0.6482\lib\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.Core.dll" to "bin\Debug\Xamarin.Forms.Core.dll". Exceeded retry count of 10. Failed. my_project C:\Program Files (x86)\MSBuild\14.0\bin\Microsoft.Common.CurrentVersion.targets 3962

Why am I getting these errors?